SoundCloud is this really cool new service that hosts music files really well. It makes FTPing or using YouSendIt links to send someone a music file obsolete.
My favorite part of SoundCloud is their sweet player. It shows you the waveform as it is playing, which is super cool. You can also leave comments along the timeline kind of like you can in the Viddler player. I was using an old, crappy flash MP3 player I found for awhile, but it was driving me nuts to I scrapped it. I like using their play a lot.