You're probably here for a list of good free and open source games. When I say free, I do not mean Free as in Mabinogi, with its cash shop, I mean 100% free. I will say if there's any commercial interest that will affect the average player. Use Google or Wikipedia to find these games.

    Battle for Wesnoth: Five @'s. Probably the best turn-based strategy game I've ever played, at least from a military strategy, as opposed to X-COM style. Nice fantasy setting, plenty of addons available. It is open source, and has single player and multiplayer.

    Spring: Five @'s. An awesome RTS, 3D, all the good stuff. Plays TA and its mods, with a conversion process. SP (Downloadable AI DLL's)+MP. Open Source.

    GearHead: Five @'s. A decent giant robot roguelike, GearHead and its sequel, GearHead 2, deliver on all fronts. It is single-player only, but is definitely worth multiple (randomly generated) campaign play throughs. It is open source.

    ToME: Five @'s. A very good Roguelike, it only suffers due to its similarity to almost every other mainstream roguelike, despite taking it a bit further. I recommend ToMENet's multiplayer as being fairly accurate, and the base singleplayer ToME is very good. It is open source.

    Paintball 3 Extreme: Five @'s. A very good 2D tile-based Zooptek game. Though it can be glitchy, it generally delivers a nice play experience. It has single player against bots and multiplayer (no bots) support. It's rare, though, and hard to find.

    Paintball 2: Four @'s. A very good 3D Quake 2 based paintball simulator. It supports single-player, and multiplayer, both with bots.

    Warsow: Four @'s. One nice FPS, that's for sure. It's got Cel Shading, and good everything. It also has Parkour, or Freerunning as one of its major aspects, and is in a cyberpunk/VR world. It is open source, and has single player (vs. AI, no campaign), and multiplayer (Standard Modes+Race, AI possible)

    Shadowbane: Four @'s. A good MMORPG, with castle building. It is a bit difficult to interact with players, and some of the places are ghost towns. It is multiplayer only (though you really only find n00bs on the test server), but has advertisements (easily ignored).

    Sauerbraten: Four @'s. A decent FPS, with a lot of SP support, though it really equates to Doom-style bots of different flavors. The MP, especially on Assassin Edition, is very useful.

    ADOM: Four @'s. A good roguelike, though it tends to be a bit windy and lengthy. It probably deserves five. Singleplayer only.

    AssaultCube: Four @'s. A good modern-day FPS. Supports SP Deathmatch and multiplayer. Based off of the same engine as Sauerbraten. Open Source.

    ChexQuest: Four @'s. A good-ol' advergame based on Doom. Supports everything Doom had, in a kid-friendly environment.

    Dwarf Fortress: Four @'s. An excelent game, if you can scratch its surface. SP only. Incredibly complex, God Game, Roguelike, and Simulated World in one. Eats up a lot of system.

    MegaMek: Four @'s. Battletech simulator. MP+SP. Simulates tabletop, pretty nice with everything counted.

    Gate 88: Four @'s. An excellent RTS game, with a twist. Plays kinda like Asteroids. I like it. Simple. MP+SP.

    I Wanna Be The Guy: Four @'s. Most annoying game ever, difficulty wise. SP only.

    Command And Conquer: Four @'s. The awesome first true RTS. Now free. SP more or less only.

    Privateer: Ascii Sector: Four @'s. One of the best ASCII non-roguelikes. SP only.

    The Elder Scrolls: Arena: Four @'s. One of the best early RPG's. Enjoyable, but needs DOSBox. SP only.

    Tyrian: Four @'s. Pretty darn good. Buy custom ships and whatnot. SP and MP.

    Tremulous: Four @'s. Nice Tactical FPS, with basebuilding and different sides. MP only.

    Plasma Pong: Three @'s: An simple, but fun Pong remake. SP and MP, but MP only against other player at the same location.