Vendetta Online is a multi-platform 3D space-combat MMO for Windows, Mac, Linux/32 and Linux/64. The game permits players to interact in a vast online galaxy; fighting, trading, and mining their way to success. A "twitch" type real-time combat model makes true player skill the deciding factor instead of character advancement. Massive capital-ship battles shake the foundations of the major nations at war, while others struggle to push back the encroachment of the AI "Hives". A no-strings free trial is available.
Crossfire is a multi-player graphical arcade and adventure game made for X. It has certain flavours from other games, especially Gauntlet and the rogue-like games (Nethack, Moria, Angband, and Ragnarok). Any number of players can move around in their own window, finding and using items and battling monsters. They can choose to cooperate or compete in the same world.
omega is a complex rogue-style game of dungeon exploration. Unlike other such games, there are a number of ways to "win", depending on various actions taken during play. The ways you can get your name on the hiscore board include becoming the highest ranked head of a guild, sect, college, etc., as well as gaining the most points figured from possessions and experience. The game (via the oracle) may impose some structure on your exploration, but you need not follow all of the oracle's advice. There *is* a "total winner" status, by the way.
Sphere is a RPGCE (a role-playing game (RPG) creation engine) that allows you to relatively easily put together an RPG along the lines of Final Fantasy 6 or Phantasy Star. It supports 32-bit graphics (with 256 levels of translucency) and MP3, Ogg, MOD, S3M, IT, and XM files for audio (and WAV and SPX (speex) for sound effects), and does not place arbitrary limits on map, tile set, sprite set, or script sizes.