I've been using Pandora for at least a year and a half now. I love it. They have a pretty good selection of celtic and Americanna. It's great how the internet has opened up everyone's musical horizons. It wasn't that long ago that everyone was stuck listening to what the local radio stations would play.

Pandora is very easy to use. You basically set up radio stations by picking an artist or song and Pandora plays songs with similar characteristics. They come up with a ton of material you have never heard of before. They don't sell the music but you can get artist and album info. Each station you set up can then be steered toward more music you like by rating songs and artists or adding more artists. I generally don't change my stations around too much because I find most of Pandora's choices are very good.

I have about 20 stations set up. Its nice to just put something on in the background as I surf x-marks. Depending on my mood, I have stations ranging from Loreena McKennitt to Steve Vai to Squirrel Nut Zippers to Rainbow/Dio/Sabbath to Glenn Miller to Forked Deer Radio.

Best of all, its available free.