Though before serious games were few and far between, today the GNU/Linux gaming market is looking increasingly varied and growing everyday. Among the major distribution platforms that offer an increasing catalog of GNU/Linux games are Steam and GOG.
Steam has brought a lot of new games to GNU/Linux. It does technically use a form of DRM. However, users can play their games offline by toggling an "offline mode". Steam has expanded to offer non-gaming software and media as well. As of this writing, they are listing 850 games for GNU/Linux.
GOG distinguishes itself for offering exclusively DRM-free media. They began by offering older classic games wrapped in a custom-emulator to make them playable on current platforms. They now offer both classic and newer games as well as downloadable movies. As as this writing, they list 165 games for GNU/Linux.
Desura is another distribution platform that offers games for GNU/Linux. It offers both DRM-laced and DRM-free games and focuses on indie developers.
Emulators are another option for gaming on GNU/Linux, especially for older games.
FLOSS games are also freely available, though these are numerous to list here. However, I will provide a short list of what I consider to be the major FLOSS games available below. It’s undoubtedly a very subjective list, but I will try to list at least one game from each major category.
Linux Game Database maintains a much more expansive and frequently-updated list of games for GNU/Linux.
Strategy & Turn-Based Games
The Battle for Wesnoth is a “turn-based tactical strategy game with a high fantasy theme, featuring both single-player, and online/hotseat multiplayer combat”
0 A.D. is historical Real Time Strategy (RTS) game. As the leader of an ancient civilization, you must gather the resources you need to raise a military force and dominate your enemies. it has been in development for over a decade and continues to be developed.
MegaGlest is a real-time strategy game where you control the armies of one of seven different factions: Tech, Magic, Egypt, Indians, Norsemen, Persian or Romans. It can played offline in single-player mode or online with other players.
UFO: Alien Invasion is a turn-based, squad-based tactical strategy game in the tradition of the old X-COM PC games with military realism and hard science-fiction.
The Ur-Quan Masters is a a sci-fi adventure game where you control a starship and try to set humanity free from alien enslavers and save the universe.
Freeciv is a turn-based empire building game based on Sid Meier's Civilization series.
FreeCol is a turn-based strategy game based on Sid Meier's Colonization.
FreeOrion is a turn-based space empire and galactic conquest game similar to Master of Orion.
OpenTTD is a a simulation game based upon Transport Tycoon Deluxe. The goal is to build your transportation empire and beat your competitors.
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.”
PlaneShift is a 3D fantasy online Multiplayer Role Playing Game.
Speed Dreams is a 3D motorsport simulation and racing game.
Stunt Rally is a stunt and racing game with 167 tracks and 34 sceneries. It focuses on closed rally tracks with possible stunt elements (jumps, loops, pipes)
FlightGear is a flight simulator, similar to the Microsoft Flight Simulator.
Armagetron Advanced is a Tron-inspired game where you control a lightcycle on a game grid.
Micropolis is the FLOSS version of SimCity Classic from Maxis (now EA). It’s a 2D top-down game with simple graphics.
Platformers & Puzzle
Pingus is a puzzle game based on the older Lemmings game. You have to guide a horde of penguins to their destination in the face of obstacles and cliffs.
Secret Maryo Chronicles bears a striking similarity to other similarly-named platformers.
PokerTH is a Poker simulation where you can play against computer opponents or online against other people.
PySol is no longer being developed, but it includes a collection of more than 200 solitaire card games and will probably run on your PC.
Rocks'n'Diamonds is an arcade style puzzle game which supports networked game sessions.
Xonotic is a fast-paced first person shooter that combines addictive, arena-style gameplay with rapid movement and a wide array of weapons.
Warsow is a fast-paced first-person shooter in the cartoonish style of Team Fortress 2 with no hard graphical violence like blood and guts.