Writing about what a cool idea Pandora is, is nothing new, but it's certainly worth writing if it gets more people to check it out. Especially given their current struggles with Washington, they could certainly use the support.
For those who don't know about it, Pandora's a site put together by the Music Genome Project which is a group working to analyze music down to its basic DNA. Sounds a little out there, but if done properly, one of the things this allows you to do is take music you already like and follow its structure to similar music that you might not yet know about, but will probably like as well.
That was the idea behind Pandora.com, a simple, well-built little Flash interface that lets you start with either a song or an artist in order to create a custom "station". Based on the DNA of your selection, Pandora then begins to stream similar music. As new songs are served up, you can either give them the thumbs-up or down so that they can keep crafting the station to suit you even more. You can create your own account and then keep adding songs/artists to your existing station or create entirely new stations around different genres. I started a jazz station about a year ago and I rarely give a song the thumbs-down these days.
The nice thing about these folks is that they keep improving on something already great. Each song that comes up has a menu that allows you to bookmark it or even link directly to iTunes or Amazon to buy it. One recent addition that I like is Pandora's stab at building an online community. I haven't completely explored it, but one cool thing I found so far is that you can take the song that's currently playing and find out who else is listening to it and what other types of music they're into.