Sony Ericsson k750i. It stores 1GB of music at the moment (though I can make it store 2) and runs the DOOM RPG, so I'm perfectly happy with it. It does, however, crash if it tries to play an mp3 file that's a bit dodgy or corrupt, but that's understandable.

It's on O2 Pay and Go, which I've been on for about seven years though I'm currently on my third telephone number.