An open source clone of the game Worms in which you manipulate a team of characters (usually earthworms) to box, pummel, blow up, and otherwise mangle the characters of the other team in a cartoon-like fashion. Network/deathmatch play possible.
Multibit is a lightweight Bitcoin client written in Java and known to run on Windows (32-bit and 64-bit), MacOSX, and Linux. Source code is on Github, installers available. Lightweight - does not locally store the Bitcoin blockchain.