Basically, it looks through your MP3 listening history, and uploads it to that site, something Last.fm calls "scrobbling".
I find scrobbling all my tracks useful for a few reasons.
- I am totally and ridiculously obsessive about cataloging my music correctly in iTunes, It's a neurosis, but a harmless one. iTunes doesn't do the greatest job of providing stats on my listening habits, though. Last.fm does.
- I listen to a lot of music, but I am also sort of old and out of the loop as far as music stuff goes - with Last.fm I can see users who have the same tastes I do, and see what other bands they are listening to and find new music that way.
- Maybe I'll make a separate post about Pandora sometime. Basically it's an Internet radio station that builds based upon a band or song that you like and your future ratings of songs it picks for you. It's very cool. Anyway, Last.fm does a good job of integrating with Pandora via a tool that someone built here. I leave Pandora on at work quietly in the background, and later I can go back and see what I listened to on Last.fm.
- I like that it is platform independent, and catches what I listened to on my iPod (walking around and in my car), at work, on my laptop... a centralized database of what I like or what I'm listening to recently. Unfortunately, right before I signed up, my hard drive crashed and took iTunes' .xml record of all my plays with it. My MP3s were backed up, but it would have been cool to have Last.fm grab my last year or so of listening as well.
- It tells me when bands I have been listening to are playing in the Chicago area. Although I think that the Early Warnings list at the Reader is still key, this is a pretty cool feature.
If you're into music, check out Last.fm. I am not a paid sponsor, just a poor writer who happened upon a cool site for people who are really into music.