Lately, Gog's policy has changed and is unable to recognize some of the great free, independent and original games that exist out there. Games who's creators have made their hard work completely free for everyone to enjoy. Personally I hope GoG eventually changes this policy as from my view point, it doesn't seem that much out of their way to give such games their own page and link as they have in the past. Negotiating with such dev teams isn't usually that difficult as long as development is active.

Anyway, failing that, there should at least be a thread listing at least some of these amazing projects so others may see it and give them a try. So here it is. Feel free to add any others to the list, but here of some of the better ones that come to mind. These games, and their assets, are free for anyone to download. Most of the titles listed (except where specified) run on Linux, OSX and Windows
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0 A.D. - Ambitious 3d RTS featuring the great ancient civilizations

Alephone - Open Source Engine for the classic FPS games Marathon, Marathon 2 and Marathon Infinity which were all released for free by Bungie in 2000

Armagetron Advanced - A classic tron light cycle game in 3D. Simple, yet addictive

Battle for Wesnoth - Really fantastic fantastical tactical strategy game

Cataclysm Dark Days Ahead - Post apocalyptic rogue like which shares Rogue's wonderful ASCII graphics

Celestia - Neat space simulator with a focus on space exploration

FlightGear - Free Flight Simulator

FreeCiv - Open source civilization game similar in scope to classic Civ2

FreeCol - Open source version of one of Sid Meier's other classics Colonization

FSO - A space combat engine derived from the Freespace 2 sourcecode. The assets for Freespace 2 are not free (although can be obtained on GoG), but there are a couple full games derived from the engine which are: Diaspora, Babylon Project and WC Saga (technically its own fork of the engine)

HedgeWars - A very complete worms style game with hedgehogs

OpenRA - Open source engine for the classic Command + Conquer RTS games

OpenTTD - Open source continuation of the simulation classic Transport Tycoon Deluxe

RedEclipse - Very fast paced arena shooter with parkour mechanics

SpringRTS - Really just an RTS engine, but there is a large community of great games that can be downloaded as part it

Ur-Quan Masters - Open Source Star Control 2 released to the community by the original developers.

SuperTuxKart - Mario Kart style racing game with FOSS mascots

Warzone 2100 - Also formerly a commercial title. A neat RTS with highly customizable units

Xonotic - Fork to a previous project named Nexiuz, its a mature quake style arena FPS based off of a heavily modified quake engine

Windows Only
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Orbiter 2010 - A great realistic space simulator with a focus on piloting spacecraft

Some fun games on the list. You might consider adding them to this thread:

https://www.gog.com/forum/general/free_games_and_3rd_party_mods_the_updated_list

What are these latest changes?

What is this list...
You lack cataclysm dda, dwarf fortress and auroa 4x????

I don't think I can consider you a gamer now.

Thanks for the info!

I'm checking out o AD :D

I'm sorry but what are you talking about? GOG have several free games in their catalog but it has never been a hosting page for freeware games.

What InkPanther said.

BTW, there's already a thread where people posted permanent free games (and DLC), although no one maintained a list in it anymore or curated the "best" games.

A small collection of collections of free games:

GOG | 17 free games
Fireflower Games | 19 free games
Truly "Free" Steam Games
The Free Bundle | 50+ free games

Marathon Rubicon: An unofficial fan-made sequel to the original Marathon Trilogy. It's IMO one of the few stand-alone total conversion mods that actually shoots for being authentic in extending, and referencing the classic lore and atmosphere of the previous Marathon games.

Marathon Eternal: Another attempt to create a "Marathon 4", but almost single-handedly created by one developer. The game is a little rougher and less polished than Rubicon, but what it lacks in polish it makes up for in ambition, and scope; in almost Doctor Who-like fashion, the player is put on an epic time-travelling quest sending them to places and circumstances interwoven with events taking place during the previous three games, and even further back before Pathways into Darkness.

Marathon - Tempus Irae: A very strange total-conversion mod that sends the Marathon universe into Renaissance-era Italy, involving a shadowy alien and Leonardo da Vinci. There's a wealth of puzzle-solving and combat, with some incredible level-design that pushes the 2.5D Marathon/Aleph One engine to its limits.

The Plan is a short and beautiful 2D insect sim / exploration game. Recommended!

[url=http://www.chiark.greenend.org.uk/~sgtatham/puzzles/]Simon Tatham's Portable Puzzle Collection[/url] is my most played (set of) game(s) over the years.

Open Sauce and available on many platforms. The Android port is particularly well done, IMHO.

OpenRa has come along way over the years.

http://www.openra.net

Thank you for compiling this list, herecomethe2000! Also to everyone adding suggestions!
Adding this to my favorite threads.

Battle for Wesnoth is definitely a must play.

I wasn't aware of most of your other recommendations but they all seem worth checking out.

Zak McKracken - Between Time and Space: Probably the best fan-made adventure ever.

Doom, the Roguelike: Well, the title says it all.

The Spirit Engine / The Spirit Engine 2: Great sidescrolling RGP/adventure games with an interesting real-time combat system.

I hope it's better than doggie doo... :\

JK, great game. :)