0 A.D. is a free, open-source, cross-platform real-time strategy (RTS) game of ancient warfare. In short, it is a historically-based war/economy game that allows players to relive or rewrite the history of Western civilizations, focusing on the years between 500 B.C. and 500 A.D.
Armagetron is a multiplayer game in 3d that attempts to emulate and expand on the lightcycle sequence from the movie Tron.
Dungeon Crawl Stone Soup is an open-source, single-player, role-playing roguelike game of exploration and treasure-hunting in dungeons filled with dangerous and unfriendly monsters in a quest to rescue the mystifyingly fabulous Orb of Zot.
Explore other star systems. Earn money by trading, carrying passengers, or completing missions. Use your earnings to buy a better ship or to upgrade the weapons and engines on your current one. Blow up pirates. Take sides in a civil war.
Freeciv is an open-source, turn-based strategy game based on Civilization II. Inspired by the history of human civilization, the game commences in prehistory and your mission is to lead your tribe from the stone age to the space age...
Freedoom is a WIP a complete free content game based on the Doom engine. It's not an engine, but it's replacement game data with entirely original content, Compatible with a majority of game mods made for Doom. Comes in Three Flavors~.
Hedgewars is a turn based strategy, artillery, action and comedy game, featuring the antics of pink hedgehogs with attitude as they battle from the depths of hell to the depths of space. As commander, it's your job to assemble your crack team of hedgehog soldiers and bring...
MegaGlest is an open source 3D-real-time strategy game, where you control the armies of one of seven different factions: Tech, Magic, Egyptians, Indians, Norsemen Persian or Romans. The game is setup in one of 14 naturally looking settings, which -like the unit models- are...
Luanti is a near-infinite-world block sandbox game and a game engine, inspired by InfiniMiner, Minecraft, and the like.
NetHack is a single player dungeon exploration game that runs on a wide variety of computer systems, with a variety of graphical and text interfaces all using the same game engine. Unlike many other Dungeons & Dragons-inspired games, the emphasis in NetHack is on discovering...
OpenRA is an open-source reimplementation of Westwood’s 
Command & Conquer Red Alert game engine, updated to use the hardware acceleration of modern video cards using OpenGL.
An open-source clone of the classic game 
Transport Tycoon Deluxe . It attempts to mimic the original game as closely as possible while extending it with new features.
Red Eclipse is a fun-filled new take on the first person arena shooter, built as a total conversion of Cube Engine 2, which lends itself toward a balanced gameplay, with a general theme of agility in a variety of environments.
Shattered Pixel Dungeon is a Roguelike RPG, with pixel art graphics and lots of variety and replayability. Every game is unique, with four different playable characters, randomized levels and enemies, and over 150 items to collect and use.
SuperTux is a classic 2D jump'n run sidescroller game in a style similar to the original Super Mario games. Instead of Mario, the hero in this game is Tux, the mascot of the Linux kernel.
SuperTuxKart is a free 3D kart racing game; we want to make the game fun more than we want to make it realistic. You can play with up to 4 friends on one PC, racing against each other or just try to beat the computer (a network multiplayer feature is planned).
Teeworlds is a free online multiplayer game, available for all major operating systems. Battle with up to 16 players in a variety of game modes, including Team Deathmatch and Capture The Flag. You can even design your own maps!
The Battle for Wesnoth is a turn-based tactical strategy game with a high fantasy theme.
Join "The Project" to restore humanity's synaptic-link technology after nuclear war has devastated Earth, rendering it a Warzone 2100.
Xonotic is a free (GPL), fast-paced first-person multiplayer shooter that works on Windows, OS X and Linux. The project is geared towards providing addictive arena shooter gameplay which is all spawned and driven by the community itself.
