I prefer Rockbox myself:
http://www.rockbox.org/
It takes a little bit of technical knowhow to setup and use, but the documentation is great. It supports a lot of other mp3 players as well and just about every kind of iPod. It comes pre-loaded with Doom and a bunch of other games.
If you look around you can find some great looking themes for it too
Good bonus is you can easily revert back to the original iPod whenever you like.