A lot of web apps seem to be targeted at problems that nobody actually has. SoundCloud is smarter than that, focusing on a problem we have at least once a week: what’s the best way to send a song to a friend? E-mail is a pain, you might not have ready access to an FTP server, and those weird Rapidshare-style upload sites can be more trouble than they’re worth.
SoundCloud basically does two things: it sends music, and it receives music. It’s the ideal setup for sending around demo copies of that new album you just finished mastering, or sharing the latest hit single (that you would never pirate because you’re a law-abiding Internet citizen). There’s great control over email notifications for new tracks sent to you and new comments on your songs, and you can play a song right from your SoundCloud account before you download it.
In terms of comments, the SoundCloud system is awesome. You can drop in a note at a specific point in a track — Viddler users might be familiar with this feature for videos — and follow your stats to see how many people are listening to your stuff. This just scratches the surface of SoundCloud’s features, so be sure to check it out and apply for an invitation if it sounds interesting to you.