Also, if you havent already, switch to Firefox and get the no-script add on. You might also like one called (I think) dont track me, and ad-blocker.
You are thinking of the extension Ghostery (for "dont track me"). I've got that one installed and it's pretty informative.
AdBlock Plus is pretty useful too, as well as Flashblock for those of us on bandwidth limited connections.
With all the hating on Google this might be unwelcome, but I just love me some Google Dictionary. This fetches a definition of anything I select on a page. Pretty nice
This all runs on Chrome too.
There is a new one called DO NOT TRACK PLUS. I run it in addition to Ghostery... seems to work well... catches most of the ones Ghostery lets through.