The only music apps I've used are ones for streaming music (Rdio, TuneIn Radio, Songza).
CopyTrans is supposed to be a good iTunes alternative.
http://www.copytrans.net/copytransmanager.php
As for players, I haven't used any because I'm fine with the native one. I've heard good things about iAlbums from music buffs who want more details about the music on their phones:
http://itunes.apple.com/us/app/ialbums/id449814696?mt=8