A 100% compatible replica of the famed SB 1.0 sound card. 8-bit ISA card. Full bill of parts. Even has a hex dump which turns an Atmel 89S51 into an 80C51 DSP chip (if you don't feel like buying one from China).
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.
OSclass is an open source clone of Craigslist that aims to be simple and easy to set up and administer. It is claimed that it has a management back-end that's even easier to work with than that of Wordpress. Written in PHP, uses MySQL. Themeable, has a plugin system.