I mentioned Collusion in a previous post. This is a plug-in for Firefox that shows all the websites that track where you visit and serve up ads based on what you have indicated an interest in.
This site http://www.aboutads.info/choices/ lets you opt out of these third party tracking sites.
The blank circles in this screen shot represent many of these sites that would be currently tracking me and creating ads based on sites I have visited and links/content I have clicked on.
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