Oh, I use it for tracking what I listen to through Pandora, Yahoo Music/LAUNCH. They also have a pay service where you can listen to pretty much anything they have in their catalog -- similar to Rhapsody or Yahoo Music. I have been tempted to get two services like that because I am noticing that other services have things YM doesn't have an vice versa. I don't buy CD's any more and because my job allows me to listen to music while I am working, I listen to anywhere from 30 minutes to a full 8 hours of music a day. Subscription services are made for people like me ...
Here is a few examples for me ...
Recent tracks listened to ...
Weekly top artists ...
They also chart your listens in a rolling 3 month format which I find extremely useful. You can use Last.fm with a lot of things including an iPod and just about any piece of software you listen to music through.