Been using Pandora since Neil turned me onto it last year and really love it. Great way to find good music, and fascinating how the "Music Genome Project" sounds like a simple analogy at first but then goes really deep and makes a lot of sense as the ultimate recommendation engine.
Stumbled onto their official blog while playing with the new Backstage feature and it led to a trail of interesting bits on how it all works. The podcast below is especially good.
Pandora's Box (article)
Founder Tim Westergren interviewed on Behind the Net (podcast).