Do you work in, or closely with, music? Do you regularly find yourself trying to send music files to people, and having difficulty due to the multi-MB nature of most music files? Yeah, I know, email sucks – but so does linking someone to a MySpace, especially if it’s a file that you don’t necessarily want on public release.
Or perhaps you’re a DJ, and you want to share mixes with people you know. Not publicly, just to close friends, or people on a mailing list. In either case, what you need is a recently-launched site called Soundcloud.