I very rarely promote video games, but I just found a great free one. It’s called World of Padman. It was originally a Quake 3 add-on map that eventually became its own stand-alone game. And boy is it ever fun. The premise is that you are apparently small blue smurf-like creatures in a the real (normal-sized world). As such, the beatifully rendered worlds are extra large. Imagine hopping onto a kitchen counter, hiding behind a toaster, and then blasting your wife (sorry Taubah). It’s not bloody or anything like that. The weapons are all quite silly.
Padman is available for GNU/Linux, Mac OS X, and yes, even Windows. The source-code is released under GNU GPL.