ones i use/have used
soundcloud is great, hosts high quality, 320 kbps...but you are limited by amount of time, not mp3 quality
soundclick, 128 kbps quality...can get annoying with anyone and everyone promoting something trying to join your friends list, no limits
reverbnation, again 128 kbps...annoying emails asking you to submit to some gig in Farawaysville USA, no limits but cannot take songs over 10mbs
dropbox, another 320 kbps site...but more for file sharing, no fancy player..limited to 2 gigs worth (that'll take a while to fill)