The GOG download is already updated to the latest official patch, including the SMAC Win 2000/XP Update. Here is a thread for fixing PracX resolution issues. Some may have been fixed since 2019, but I don't know.
https://www.gog.com/forum/sid_meier_s_alpha_centauri_/left_mouse_crash_when_using_pracx

All patches were available at http://alphacentauri2.info/
With AC2 down, this page was last updated with a link to githubs, Thinker mod and DOS days mirror on November 25, 2024, prior update October 31 2022. Scient's unofficial patch for the expansion only did bug fixing if you want nothing else. I personally think you are missing out if you never try AI improvement mods.

Smac-in-smacx farther down allows you to use mods and unofficial patches made for the stand-alone expansion in a game largely akin to the original. It is not necessary for the Thinker mod by setting smac_only=1 in thinker.ini

https://github.com/DrazharLn/scient-unofficial-smacx-patch/
https://github.com/DrazharLn/scient-unofficial-smacx-patch/releases/download/v2.1/SMACX_UP_v2.1_Installer.exe

Here is a link to "DOS Days" website. It offers Scient, Kyrub and Yitzi patches. Scient is bugfixes, Kyrub improved AI and incorporated an earlier Scientt. Yitzi incorporated Kyrub and an earlier version of Scientt. Induktio's mod farther down is more recent, and probably has "the most extensive AI improvement this game has ever had", but includes other modifications.https://dosdays.co.uk/topics/Games/game_alpha.php?fbclid=IwAR1ykNPGk5tMY72lkyw6OINqHc6utxUzdeLOd5g2CSiFbxZtnjM7lqDjlng#AF

Yitzi is more extensive than kyrub and may not work for all Mac users. if Induktio's or Yitzi's changes don't work for you, I would still encourage you to try Kyrub some time. Including earlier scient bugfixes, Kyrub's AI improvement was still extensive. Dos days mirror is largely up to date to when I had previously reviewed the game, but has kyrub version "K". Kyrub has a version "P" in a still functioning post below mine. Alphabetically, P may his last update.

Induktio's 2023 thinker mod page has links to some other downloads including smac-in-smacx. If not for some additional modifications I would have just considered it unofficial patch. Although previous contributed a great deal to bug fixes and AI development, Induktio is the more recently updated mod, if you're up for his change some time. AI growth mod farther down is a formal rebalancing mod.
https://forums.civfanatics.com/threads/smacx-thinker-mod.682321
https://github.com/induktio/thinker

Apart from an inclusion of bugfixes (including scient), Induktio's thinker mod focuses on AI improvements, and improving map generation (which can be disabled). It modifies combat with an intention to improve it, including reduced healing, and helicopter range. He seems to believe that Helicopters had an inordinate advantage compared with higher-tech jets. The mod enables colonies to start with more buildings after a certain tech level. According to Induktio's notes, this was potentially going to be included in the original game, since it's in the files.

For something closer to the original resolution, set to window_width=1024, window_height=768 in thinker.ini

Civ fanatics is also hosting AI growth mod (2024), which is formally a rebalancing mod, and not just a patch. AC2's site ("Uncle Buster") had a number of graphical and custom faction mods, but I am afraid they've never been my focus. Although I have not played it yet, I am including this mod since, apart from Induktio it's probably the major mod of our time, and likely the only other mod updated recently at least in CivFanatics files.
https://forums.civfanatics.com/resources/smacx-ai-growth-mod.31053/

If you are having a problem running AC, try solutions mentioned in these thread.
https://www.gog.com/support/sid_meiers_alpha_centauri/bgeneral_troubleshooting_guide_mac_os_x_gamesb
http://www.gog.com/forum/sid_meier_s_alpha_centauri_/terranexe_has_stopped_working/post13
If you need my attention, you'll need to private message me. I don't visit this forum every day.

===Unofficial patches introduction===
The primary intention of this page was to help the user to understand and correlate a variety of unofficial patches, some of which are still available on this thread with AC2's site down. They are listed by date of release. Apart from my update listed above, I had last attempted to update the table in 2022 before AC2 went down. Some information may be out of date, but probably not.

Table of Conents
Patches -Bugfix
Scient's SMAC/SMAX (2010-2014/2019)
Bugfix/AI improvements
Kyrub SMAC (2010-2014/2015?)
Kyrub SMAX (2012)
Ytizi SMAX (2012-2017?)
Dio's SMAX (2015/2019)
SMAC-in-SMAX (updated 2019)
Mods
SMACX Thinker Mod (2018-2023)
SMACX AI Growth mod (2018-2024) (formal mod with rebalancing)
SMACX The Will to Power (2020-2022?)

===Unofficial patches detail===
The patches are listed here chronologically and are also listed at this location.
http://alphacentauri2.info/index.php?action=downloads;cat=2

Scient's patch (2010-2019)
1) Originally released in 2010, Scient's patch fixed a number of crashes, bugs, and exploits. V1 removed the unit limit but this was reversed in later and other patches due to crashing large games. Originally for the expansion, it is now available for both games. It was updated to 2.0 in 2014, but was incorporated into Yitzi's patch and not released separately until 2018. It was finally updated in August 2019 to version 2.1.
http://alphacentauri2.info/index.php?action=downloads;sa=view;down=365
http://alphacentauri2.info/wiki/Scient%27s_patch

Kyrub SMAC 2010-2015?
2) Released in 2011, this patch uses Scient's 2010 release. The latest 2014/2015 release, SMAC 444 (version _p) for the original game (SMAC), adds bug fixes to the game and experimental alterations to the AI intended to improve it. The AI improvements may be considered the primary feature of the patch.
http://alphacentauri2.info/wiki/Kyrub%27s_patch
http://alphacentauri2.info/index.php?topic=8703
http://www.gog.com/forum/sid_meier_s_alpha_centauri_/another_smac_fan_patch/page3

Kyrub SMAX 2012
Kyrub ported a number of the fixes he made for his SMAC patch to a patch for SMAX, the Unofficial patch PLUS. This patch hasn't been updated to his latest SMAC 444 patch, and the AI only has some improvements related to terraforming.
http://alphacentauri2.info/index.php?action=downloads;sa=view;down=108

Ytizi SMAX 2012-2017?
Started in 2012, Yitzi's patch for the expansion (SMAX) builds on Kyrub's SMAX patch, but reportedly incorporates Scient's 2.0. It was last updated to v3.5d in 2017. Besides a number of it's own additional bugfixes and inclusion of Kyrub's AI enhancements from that patch, it also improves the modability of the game.
http://alphacentauri2.info/index.php?action=downloads;sa=view;down=352
http://alphacentauri2.info/wiki/Yitzi%27s_patch

Yitzi's patch now includes PlotinusRedux's PRACX for resolution changes. This can also be downloaded separately at alphacentauri2.info.

Dio's Unofficial SMAX Patch Version 1.1 2015/2019
http://alphacentauri2.info/index.php?action=downloads;sa=view;down=321
Initially released in 2015. Requires Yitzi's patch. "enables various features associated with probe teams having a green morale level". Dio's initial contributions to bugfixing were apparently incorporated into the main Yitzi patch, but Dio's patch was updated again in 2019.

SMACX Thinker Mod 2018-2024
https://forums.civfanatics.com/threads/smacx-thinker-mod.682321/
https://github.com/induktio/thinker
https://alphacentauri2.info/index.php?topic=21013.0
Released in 2018, this AI mod/improvement includes Scient's 2.0 patch and states that is compatible with SMAC-in-SMAX.. It also attempts to balance starting faction placement, and makes it possible to select which landmarks are placed on random maps.

Will to Power refers to the AI in this patch as shockingly smart.
Creator invites and requests bug repots directly to the github rather than the AC2 forum

SMAC-in-SMAX updated 2019
https://github.com/DrazharLn/smac-in-smax
"These files let you play an only mildly alderated Sid Meier's Alpha Centauri in the Sid Meier's Alien Crossfire engine. You might want to do this to benefit from various unofficial patches for Alien Crossfire."

SMACX AI Growth mod 2018-2022
https://alphacentauri2.info/index.php?topic=20959.0
Released in 2018, this mod includes AI and other improvements, but also modifies the game in other ways including an attempt to round out the factions. The mod continues to be updated at time of this page's update, October 2022.

SMACX The Will to Power 2020-2022
https://github.com/tnevolin/thinker-doer/releases
https://alphacentauri2.info/index.php?topic=21359.0
Includes Thinker Mod.
Includes a revamping of unit/cost/combat system, favoring defenders but with more randomness.
Modifies the technology tree.

===Miscellaneous===

I also wanted to post Alpha Centauri's backstory (fiction) here, which isn't currently in the extra's section of the gog download page. Originally on the official page before it went down, an archive of the site was reposted on //alphacentauri2.info.
And I wanted to recommend the research assistant on the second link.
http://alphacentauri2.info/official/Sid%20Meier%27s%20Alpha%20Centauri%20The%20Story_cfm.htm.
http://www.civgaming.net/smac/misc.shtml

The following is a tool for editing the cvr files in Alpha Centarui. It allows you to paint the various parts of the units different colors.
http://alphacentauri2.info/index.php?topic=7866
http://www.gog.com/forum/sid_meier_s_alpha_centauri_/new_tool_for_modding_units

Translation to French
http://www.gog.com/forum/sid_meier_s_alpha_centauri_/fr_alpha_centauri_une_traduction_complte/page1

Troubleshooting
http://www.gog.com/forum/sid_meier_s_alpha_centauri_/finally_fixed_win7_crash_terranexe_has_stopped_working_fix
http://www.gog.com/forum/sid_meier_s_alpha_centauri_/fixes_several_issues_windows_7
http://www.gog.com/forum/sid_meier_s_alpha_centauri_/game_crashing_on_mac_terranxexe_issue

Major update of SMAC_444 patch, about 70 assembly hits for a ruthless SMAC AI.
Download link

The patch does *not* work with SMAX (I am sorry, I had/have no time for it; I have not got even the time to finish this).
The patch includes scient's latest patch, fixing a good number of bugs.

Features
- AI will stand up and fight in air wars. It builds a lot more AAA defenders, it uses planes for other things than bombing formers. You may see it use hit and drop tactic.
- AI will attack with while in "hurry" (with 2/3 strength etc.). It should counterattack from bases more.
- AI sees enemy probes as a primary target and may go long way to destroy them.
- AI uses more intelligent pattern for city spreading. AI colpods will now rarely walk half map somewhere.
- AIs ruthlessly exchanges technologies between themselves, hopefully this makes them competitive until later stages of the game.-
- AI likes buildings a lot more. It is able to rebuild its hedaquarters now. It loves orbital pods.

+ [older features]

This update was untested, only for crashes. I saw just 1 test game, the early signs were good, but who knows.
I'll be glad to hear reactions and feedback.

Thanks for working on SMAC kyrub, gonna start a new game today so I'll load up the patch.

I'd say the change is mildly dramatic.

For those of us knew to SMAC/X, could you advise which unofficial patches are pure bug-fix patches, ie. don't add any new features? At the moment only (1) seems to fit that bill, strictly speaking, unless the AI improvements in (2), (3) and (4) correct glaringly poor AI decision-making without going overboard. (5) would seem to be right out.

Also, if we install (1), which additional patch should be applied to correct the crash caused by the unit limit removal?

Kyrub's patches don't add any features (besides bugfixes) to the game other than AI improvement. Yitzi has begun including bugfixes which I would consider to really be modifications but I don't personally consider the modification extensive at this point to be of worry for anyone who might not want modifications.

Scientt's patch is the only one that tried removing the unit limit, but you could probably install it if for some reason you don't want the AI improvements. Your game would only crash at 2000+ units on the map. Seeing as there were recently a lot of people using it, it's entirely possible you won't run into this.

I believe all of these patches are just EXE files. There aren't any installations.
If you want or don't mind AI improvements, and want bugfix SMAC, get Kyrubs patch.
If you do not wants AI improvements, Yitzi's SMAX patch is built on Kyrub's SMAX patch, which only has improvements to terraformer AI.

Scientt's foundational patch for SMAX, which all the others are built on, is outdated in bufixes and also has no AI improvements.

Which of the unofficial patches are more stable when it comes to running it in widescreen? (1080p)

I bought the game yesterday actually, merely wondering which was recommended since I plan to use the DirectDraw=0 command to get desktop resolution.

outdated post

hmm, i always get a "server not found" message when i try to get one of the unofficial patches.
Is the alphacentauri2 website down?
Is there any other possibility to get the unofficial patches?

I'm not having any problems. Are you still having problems? Private message me.

Yeah, the whole alphacentauri2.info seems to be offline. Last night the main page had some errors, today there's some content but not much. So all the links are dead right now. Sucks, 'cause I was in the mood just today.