I just installed Lightbeam for Firefox. It's a new add-on that tracks and displays the third-party sites you interact with on the Web. Actually, I should say, "that interact with you" because these are in-the-background interactions that you don't initiate or even know are happening.
So, I browsed six sites over 10 minutes (researching tile grout sealer—exciting!), and then checked Lightbeam. It showed that 94 other sites had observed my browsing. Ninety-four. To the left is the Lightbeam graph: Circles represent sites I went to, triangles are the 94 others.
What with the NSA listening in on Angela Merkel's phone chat (since 2002!) I guess I shouldn't be surprised that a few entities were observing my Web behavior. But still.
Go get Lightbeam and try it out. See who's watching you. --Bill Paarlberg, editor