The Internet is free, but you're not free on the Internet. There are ads and cookies and all manner of things which can track your activity online. Mozilla's ethos has always involved maintaining an open web; now they're going to show you just what shadows you online.
The Collusion website is an interactive explanation of how it works, taking you through some of the net's most popular sites. The IMDb, New York Times and Huffington Post are three of the examples, and they give a chance for you to see what ad-providers are following you, and where they're doing so.
I looked at a couple of automotive websites and a few games websites and you can see the result in the above image; it builds up a picture of just how quickly your human presence online becomes a valuable tool for market research. As an added bonus, you can export the entire network of links you're tied to.
Collusion is available from the Mozilla Addons website, and it doesn't require a restart to work either. As a Mozilla project it might never progress to a fully-fledged point, but even now the concept is solid enough to be worth a look.