Spotify's got a huge library of records, and you get access to all of it. And it's a lot of stuff, from normal albums to rare tracks, which you get to stream at will, by album or single track. Or pick a genre, and decade and the program will make a continuous playlist for you of for example jazz released between 1970 and 1990.
You can get the free version, which works kind of like a commercial radio station, or the premium version without the ads. I'm hoping they succeed with their business venture, because it's a really excellent product!
EDIT: Although, I do find a few holes in the catalogue the harder I "test" it - no Meshuggah, that's kind of bad!
