FreeTumble is yet another "destroy adjacent same-color bricks" puzzle game but one of the better-looking and -sounding ones. Three modes of gameplay are provided, they can be seen as variation of the same game for different types of puzzle players or different hardness modes.
Biniax-2 is an original puzzle game in which colors also play a role and which also is stronger on the visual side than your typical open source Tetris clone. It features a turn-based and continuous mode. The latter can be played with two players in a versus-game.
Simon Tatham's Portable Puzzle Collection will help you if you're bored at the office or similar.
CreepSmash is a java-based multiplayer tower defense game. Have some videos of it too!
26 June 06:09 UTC PS: RubyWeekend #3 with the theme "A Tiny World" started a few hours ago. It's a 3-day open source ruby game programming competition.