The Last.fm Scrobbler for Chrome detects the category of the video you're watching and tries to find the artist and the song title, then it sends this information to Last.fm. The first time when it scrobbles a song you'll see a Last.fm page that requests your permission.
The extension shows notifications when the track changes and it supports many other sites: Google Music, Pandora, MySpace, Google+ (the YouTube widget) and more. At the moment, you can't scrobble tracks in Google Music because Google changed the URL, but an update should fix this issue.
{ via Matthew }