An open source utility for Windows (32 and 64 bit) that will securely erase files and directories on a Windows machine, in addition to scrubbing free space and OS-related temporary files that could leak information. It can even wipe an entire system if you need it.
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.
A realtime strategy game in which you compete in a virtual space with others to build the biggest, most robust network you can while attempting to compromise or crash the networks of rival system admins. It sounds a little like Starcraft meets Neuromancer with a healthy dose of the movie Hackers thrown in for fun. Runs on Windows as well as Linux.