I’ve been looking for a good way to scrobble my iOS device music plays to my last.fm account. Both Rdio and Spotify have built-in scrobbling, but if you don’t use either of those services then you should check out Cloudscrob.
The thing that makes Cloudscrob stand out from the rest of the crowd (like iScrob, which I think is actually the developer’s OLD scrobbling app) is that it realizes that the built in Music application is already pretty good. It doesn’t try to replace it. Just launch Cloudscrob after playing some songs and it submits those songs to Last.fm.