Lightbeam is a Firefox add-on that enables you to see the first and third party sites you interact with on the Web. Using interactive visualizations, Lightbeam shows you the relationships between these third parties and the sites you visit.
Here is a little more about Lightbeam from the developers:
Using interactive visualizations, Lightbeam enables you to see the first and third party sites you interact with on the Web. As you browse, Lightbeam reveals the full depth of the Web today, including parts that are not transparent to the average user. Using three distinct interactive graphic representations — Graph, Clock and List — Lightbeam enables you to examine individual third parties over time and space, identify where they connect to your online activity and provides ways for you to engage with this unique view of the Web.
When you activate Lightbeam and visit a website, sometimes called the first party, the add-on creates a real time visualization of all the third parties that are active on that page. You can pick up Lightbeam from the Firefox Add-ons website.