Free Open Source Games

Games on this list were included based on recommendations, if you'd like to a game added to this list, please post it on the sticky of /f/Games. For a more comprehensive list, please see this Wikipedia page.

Adventure/RPG

  • Beneath a Steel Sky - Classic adventure game with excellent artwork and voice talent. Should be available in the Ubuntu repos
  • Eldevin - F2P Runescape clone with a distinct charm
  • Flare - A surprisingly in-depth and polished isometric RPG
  • Heroine's Quest - Fantastic free Point'n'Click RPG inspired by the Quest for Glory series from Sierra
  • Lugaru - Turner, is an anthropomorphic rebel bunny rabbit with impressive combat skills. In his quest to find those responsible for slaughtering his village.
  • ScummVM's Freeware Games - Some fantastic point'n'click games here, my favourite probably being Dreamweb
  • Solarus Games - Solarus is a game engine for making Zelda-like games, but there are many completed games available on their website, and also community made games

Arcade/Casual

City Builder/Tycoon

  • OpenTTD - open-source clone of Transport Tycoon Deluxe
  • Scorched 3D - Simple turn-based or real-time artillery strategy game
  • Simutrans - Open-source transportation simulator clone.
  • Unknown Horizons - Village builder inspired by the Anno series
  • Widelands - Polished Settlers clone

First Person Shooter

  • Alien Arena - Retro Sci-Fi Online FPS
  • ET Legacy - Wolfenstein: Enemey Territory for modern systems
  • Cube 2 - Quake-like Arena online FPS with multiple game modes
  • Freedoom - The Freedoom project aims to create a complete free content game based on the Doom engine.
  • OpenArena - Online arena FPS using the id tech engine
  • Ravenfield - Singleplayer Battlefield type game
  • Red Eclipse (GNU/Linux, BSD, Windows & Mac) - a free online arena shooter with parkour.
  • Termulous - RTS/FPS Hybrid with aliens!
  • The Dark Mod - Fan-made spin-off the the fantastic Thief series (Not suitable for low-end systems)
  • Urban Terror - Fast paced online FPS reminiscent of Counter-Strike
  • Warsow (GNU/Linux, Windows & Mac) - a fast-paced FPS in a futuristic cartoonish world.
  • Xonotic (GNU/Linux, BSD, Windows & Mac) - a free online shooter, based on Quake.

MMORPG

  • Ryzom A game where there are no classes to choose, nor force to choose a faction or guild.

Minecraft Clones

Puzzle

Racing

  • Speed Dreams - Realistic racing sim
  • SuperTuxCart - Open-source clone of the Mario Cart games (Not suitable for low-end systems)
  • Yorg - A top-down racer in the Micro Machine's style.

Roguelikes

RPG

  • Angband (GNU/Linux, Windows, Mac & Android) - a single-player dungeon exploration game in the universe of JRR Tolkien.
  • Shattered Pixel Dungeon (Android) - a traditional roguelike game with pixel-art graphics and simple interface.
  • Cataclysm DDA - Cataclysm: Dark Days Ahead is a turn-based survival game set in a post-apocalyptic world
  • Star Ruler 2 - Explore, expand, exploit and exterminate your way to become the ruler of the Galaxy.

Side-Scroller

Simulation

  • Minetest (GNU/Linux, FreeBSD, Windows, Mac & Android) - a near-infinite-world block sandbox game, inspired by InfiniMiner, Minecraft, and the like.
  • OpenTTD (GNU/Linux, Windows & Mac) - a transport simulation game based upon the popular game Transport Tycoon Deluxe, written by Chris Sawyer.
  • OpenRCT2 (GNU/Linux, Windows & Mac) - An open source reverse engineering of Rollercoaster Tycoon 2. Requires assets from the original game, acquirable here

Space Sim/Trading/RPG

Strategy

  • 0 A.D. (GNU/Linux, Windows, Mac) - a game of ancient warfare.
  • The Battle For Wesnoth (GNU/Linux, Windows, Mac, Android & iOS) - a turn-based strategy game with a high fantasy theme.
  • Freeciv (GNU/Linux, Windows) - A clone of Civilization II from the Sid Meier's Civilization series.
  • Widelands (GNU/Linux, BSD, Windows & Mac) - A real-time strategy game focused on building an economy. Based on Settlers 2.
  • FreeCol - Clone of Colonization
  • FreeOrion - Clone of Master of Orion
  • OpenRA - Open-Source implementation of C&C: Red Alert, Tiberian Sun, and Dune 2000
  • Tanks of Freedom - Turn based strategy, similar to Advance Wars on the GBA.
  • Hedgewars - Clone of the game Worms
  • UFO: Alien Invasion - Turn-based tactical combat game, inspired by the X-Com series.
  • Argentum Age - Open-source fantasy card game
  • UFO2000 - Another X-Com inspired game
  • Wyrmsun - An interesting fantasy RTS
  • Spring RTS - RTS game engine that has a few complete games made from it, like Zero-K
  • Warzone 2100 - 3D RTS that offers campaign, multi-player, and single-player skirmish modes.
  • Megaglest - RTS where you control the armies of one of seven different factions
  • PokerTH - Free online poker with the best open source texas holden poker game and a great community
  • Seven Kingdoms: Ancient Adversaries - GPL release of Seven Kingdoms: Ancient Adversaries