Soundcloud is a superb new service to exchange and distribute audio files on the web. Currently in closed beta, but try signing up for an invite, I got one within a few days. 

It works great for both private and public exchange, with a few brilliant features: Waveform overview, unlimited storage, ability to share files with other users, share files with specific email addresses, share files with everyone, group tracks in sets, a sharing widget, and audio timeline commenting. And more.

I loves it.

Here's an example of some mindless piano development noodling I did late one night, showcasing the sharing widget and the commenting feature.