GP2X Supported formats:
SID (C64 Music),
SPC, NFS, GBS, GYM, VGM, VGZ, KSS, NSFE, SAP, HES, UADE (various).
The GP2X has an excellent, very mature set of music players covering almost every format.
CraigAMP which is a winamp 'Classic' clone supporting playlists (Creating, deleting, loading, saving),
covert art, searching tags and other winamp features.
OldPlay which aims to support every music format ever (Yes even Amiga, C64, ST etc.)
GMU which is an original player for the GP2X featuring cover art support, playlists, FLAC decoding and more.
Due to this excellent line up of players the GP2X has one of the best music player experiances of any
handheld, plus they are open source so you can feel free to add new features or tweak them to your tastes.