Spotify
Just discovered. It's free & awesome. You can find just about any song you want and listen to it legally. It's like Pandora meets iTunes.
Spotify is cool. MOG and RDIO are similar services. After trying all three I bought a subscription for MOG, mainly because I liked it's user interface and iPad app better.
In terms of selection, all three seemed equally good. It seems like if it's currently in print, you can get it through these services. The exceptions are artists that do not choose to make their music available to these streaming services. These include the Black Keys, Gillian Welch, and the Red Hot Chili Peppers. Apparently these artists feel that the terms are not fair to artists. I just found out about this when I tried to find some Gillian Welch on MOG and couldn't find much (she's not on Spotify or RDIO either). I want to read up more on this. Originally I felt good about my MOG subscription because I felt artists were still getting fairly compensated but now I'm not sure. On the other hand, I do still buy music the old fashioned way through iTunes and such. Piracy is going to be so hard to stop, so a middle ground like these advertising supported streaming services with optional subscriptions seems like a way of at least providing some revenue to recording artists, even if it's not as much as in the heyday of the recording industry.
Here's more info: http://www.theverge.com/2012/1/26/2740981/debate-spotify-mog-rdio-kill-s....
Plus I found out that I can take use the mobile Spotify app once you subscribe. I'm on it.
Be sure to subscribe rather than using the (limited) free service. For £5 a month you get uninterrupted listening (no adverts after every other song) and, as I understand it, the artists collect royalties for more listens. There's a ton of blues on there!