Mythos:
http://www.mythos.com/index.jspThis game is completely free, and it's very similar to Diablo (it's made by Flagship Studios, which was formed by the former Blizzard guys that actually did Diablo). They've just finished alpha testing, and are preparing for closed beta, so it's not exactly a full game yet. However, even when it is finished, it will still be totally free--they're basically going to use this game to test out the engine for their "flagship" game, Hellgate: London.