The Rockbox project produces an open source firmware replacement for portable Digital Music Players, currently supporting Archos, iriver and iPod models with more platforms being worked on.
I just wish they'd host their files here so I could subscribe to a release RSS. I absolutely love running this on my Sansa Fuze v2. You get about 50 free games and other useful applications. I couldn't be happier.
Any kind of effort into making like this, and adding different options than those of the producer is always nice. I'd love to have Rockbox on my PMP :)