The Hype Machine aggregates postings from various mp3/music blogs in an easily digestible form.  It's basically a Google News for music.  And it's a great service.  Great to keep tabs on new music.  It has a heavy indie music bent (not a bad thing), but mainstream artists like Gnarls Barkley can also be found.  Each listing on the site contains options to let you listen to the song, view the original blog post or purchase from iTunes or Amazon.  I searched for Belle and Sebastian, one of my favorite bands, and found several tracks I'd never heard, mostly bootlegs from shows.  I'm listening to their cover of Sympathy for the Devil as I write this (terrible recording but beggars can't be choosers).  The site offers several listening options and I used the Flash player, which kept buffering but it could have been my internet connection. 

I'm sure many of the tracks posted by the blogs that Hype Machine links to weren't authorized for such distribution by the copyright holder, but the site seems to take copyright seriously.  In fact it even does a service to such copyright holders by showing which sites are posting their tracks so they can decide whether to go after the offending site or leave well enough alone.  Business-model-wise, I'd guess it's revenue from ecommerce referrals, advertising + a strong dose of being a labor of love.  Whatever it is, it fills a need and I hope it sticks around.