I found Pandora about 2 weeks ago, and I've been loving it ever since.  Pandora is a "personalized music station" service, as I would describe it.  You tell it an artist (or artists for better results) or a song (or songs) that you like, and it will create a "Station" based on that.  You can create multiple stations.  I have several that I play depending on the kind of music I'm in the mood for.  Best of all, it does all of this for free (ad supported, with the option to pay to remove ads).

Pandora is driven by a project called the "Music Genome Project" where some people have spent extensive time breaking music down into it's elements, and then classifying a large amount of music by that criteria.  It's really an amazing result.  About half of the music that I hear is music that I own, and that shows me that Pandora knows my tastes prety well.  The other half is music that I've never heard, but most is music that I love now that I've heard of it. 

You can tell Pandora if you like a given song, and it will improve your station as you do this.  After two weeks of telling Pandora what I liked, it now plays a song I don't like only about 1 in 25, and it continues to get better every time I listen.  Beat that, 105.7 FM.

Pandora is a Flash application.  Which is actually pretty nice because it runs over good ole port 80, so it will work at the office unless your company specifically blocks the pandora.com domain. 

As the Play Station 3 supports Flash, I was hopeful I could play this on my big entertainment center using the PS3.  No such luck.  The PS3 runs out of memory before it can load the whole applet.  Ah well, not Pandora's fault.

On the subject of legality, I really hope that some authority like the RIAA doesn't come down on Pandora.  If anyone in that position is listening, I've already purchased the albums of several bands I found through Pandora.  Pandora does increase music sales by connecting listeners with music that they will like, I'm guessing.

At any rate, give Pandora a try.  You won't regret it.