Saints Row 2, The Third, IV , and Gat out of Hell - SaintsRowMods.com is an active community, and IdolNinja himself was very involved in it before getting hired by Volition, now he "just" helps the modders there :D

Heroes of Might & Magic V - the MM5.5 mod for the Tribes of the East expansion adds numerous features and is updated frequently

Worms: Armageddon - more recent patches "fuse" WormKit into the game, enabling native mod support. See list.

RollerCoaster Tycoon 1 (LL) & 2 (with and without expansions) - NEDesigns (http://www.nedesigns.com/) is a site where people still show off their parks and tracks. Loopy Landscapes is still a thing, ladies and gentlemen. Also, what Matewis said, OpenRCT2 is an open-source engine reimplementation of RCT2, and is 100% compatible with RCT1 scenarios and tracks in more recent builds, and has a new multiplayer mode - its possibilities are pretty much infinite now.

Ahw, hadn't seen that, sorry. :^(

Lot's of mods for Total Annihilation.

http://www.tauniverse.com/

I've seen that you have added Quake. Feel free to add Q2 and Q3. Both games also have new mods. Are you looking for some quality stand for the mods?

Also, Quake III has a fan-made engine, ioquake3, based on the official source code by id Software.

Darn. It looks pretty cool, but I don't see a download link anywhere.

Because there isn't one .

The classic Tomb Raider (Chronicles) modding scene is still alive and kicking.

http://www.gemrb.org / https://github.com/gemrb/gemrb
Runs the Baldur's Gate, Icewind Dale and Planescape: Torment games

http://rr2000.toshiba-3.com/pc-files
For Carmageddon 1, 2, TDR and Reincarnation. He's also made a pack that mixes Carmageddon and Splatpack into one, and it works very well.

http://dengine.net/ Doomsday engine for Doom 1, 2, Final Doom, Heretic and Hexen.

Doom 3 Hi Def. (Both original and BFG)

http://www.moddb.com/mods/brutal-doom

http://www.moddb.com/mods/penumbra-necrologue
http://www.moddb.com/mods/penumbra-twilight-of-the-archaic

http://www.markshan.com/knightmare/ Quake 2

http://www.jonhunt.com/redneck/erampage.html

Thanks for another suggestions, I've added the games to the list.

The Elite Force Mod for SWAT 4 still gets updates:

http://www.moddb.com/mods/swat-elite-force

How 'old' is 'old' exactly?

(Just wondering how 'old' games in this thread should be)

You can suggest anything in this thread.

However in my gogmix I'm considering the old as not newer than... let's say 2005.

(now I see that Heroes of Might & Magic V was released in 2006, than one slipped through!)

This fucking thread is gold. It should be stickied.

SimCity 4 has quite a few mods (one of the big ones out there is a mod that adds all sorts of new transport features to the game but there are others)

I can think of a few other older games that have mod communities but none that are on GoG (Command & Conquer series, Valve games, Blizzard games) so those aren't a candidate for this mix :)

Dark Reign
http://darkreign.ws/downloads/