I am amazed how Rockbox turns my old, rather outdated MP3 player into a MP3 player capable of playing modern codecs and running useful applications.
For example, the stock firmware on my Sansa Clip Zip wouldn't allow me to play Opus files, which I store my music library in, but Rockbox supports it and many more codecs. Even classy MOD files are supported! (Granted, the stock firmware got support for Ogg Vorbis files through an update.)
And yes, you can play Doom on it. Programming for Rockbox is pretty easy too, see this wiki page.