I use last.fm to track what music I listen to. Unfortunately, it’s not very accurate.
The first problem is that it doesn’t track music listened to on most online services (e.g. Youtube, Bandcamp). But I can’t really complain about that, since I just discovered there’s an app for that. Though, its Youtube support is shaky, I assume because it’s hard to tell what’s a non-musical video and what is a music track.
A bigger problem for me is that last.fm counts up what I listen to by counting tracks played rather than by counting time played. So if I listen to a punk band for one hour, and then I listen to Miles Davis for one hour, last.fm will make it look as though I like the punk band 10x more than I like Miles Davis, because the punk band writes 3 minute tracks and Miles Davis records 30 minute tracks.