It's a bit shameful that I haven't mentioned the awesomeness that is pandora. Sorry if this post ends up reading like an ad but I love this site.
Pandora allows anyone to create their own customizable Internet radio stations. The style of the radio station you make is determined by whatever artists or songs you choose as "seeds." The pandora algorithm selects a playlist of songs that share qualities with your seeds.
For example, I just made a radio station with the "artist" Debbie Gibson as a seed. The first song I get is Gibson's "Lost in Your Eyes," a classic ballad to warm the heart and moisten the eyes. The next song that comes up is Paula Abdul's "Forever Your Girl" off the album of the same name. Pandora tells me that it selected this song because it features easy listening qualities, great musicianship, a subtle use of vocal harmony, groove based composition, and repetitive melodic phrasing similar to those of Debbie Gibson. Not quite the description I would give but accurate nonetheless. The next song is "Material Girl" by Madonna.
You can create stations based on much more obscure groups as well. Some of my seeds include Land of the Loops, Avi Bornick, and Beam. Not exactly top 40 material.
Pandora uses the All Music Guide as a source for information on the artists and links over to iTunes and Amazon so that you can instantly buy any song you like.
If anyone already has or gets a pandora account, email me and we can share stations. Cooperation: that's so crazy, it just might work.