You have the default player in Zodiac. Its name is Music.
Aeroplayer is free for ogg and speex. You could convert your mp3 to ogg.
Dioplayer is free and play mp3 but it doesn't turn off the screen. At least not in my zodiac 2 :-(
TCPMP is an option too.
Thanks everyone. I should have noted, I was looking for one that could handel playlists. It seems like Dioplayer does, thanks again. I listen to books, they need to play in order and I was having a hard time with the zodiac built in player.