I've used GSPlayer recently and it's pretty good, a lot of settings you can play with.
I found the preamp feature especially useful, since my PocketPC (Jornada 568) outputs sound at a volume too high while listening through some headphones (those small ones that come with portable CD/MP3 players etc.), even when I have the volume settings at minimum (just before mute), so the preamp enables me to lower the music volume even more.
