Collusion for Firefox is a browser add-on that, quote:
“Collusion is an experimental add-on for Firefox that allows you to see which sites are using third-party cookies to track your movements across the Web. It shows, in real time, how that data creates a spider-web of interaction between companies and other trackers.”
Currently the add-on somewhat informative and displays a visual presentation of tracking sites, see image on the left.
The final version of Collusion will add capabilities for blocking cookies for any node in the graph. It is somewhat interesting to see the companies tracking your Internet activities, and how your activities might be shared between these companies.
The better option is to collectively block tracking altogether by other add-ons, such as DNT+, Ghostery, No-Scripts, etc. DNT+ supports most browsers, except Opera, and Ghostery supports all browsers. The No-Scripts add-ons is available for Firefox only.
If tracking your Internet activity is a concern of yours, you do not need to wait for the final version of Collusion. Other add-ons are available now and do a good job at disabling of your Internet activities tracking.