The collusion add-on for Firefox is super-legit. Just navigate to any website normally, then just click the little collusion icon in the status bar and a full detailed report pops up in a new tab describing all the sites that the current site shared your data with (shared as in connected).
View the full introduction to Collusion on mozilla.org, or view my Firefox Add-on Collection: AskApache Web Development (Advanced)
Advanced Web Development by AskApache is a Firefox Collection I created since I'm always trying new Addons out and using multiple computers and I wanted a quick and easy way to install my favorite's and keep a running list. Firebug, YSlow, LastPass, and Web Developer are the only ones I always use regularly.
I like the idea of the last.fm but it's not as powerful as the site, which is awesome. Lately listening to Kings of Leon Radio...