When you connect to a website on the Internet connections are usually not only made to the site, but also to third party servers and websites. This does not have to be a bad thing, as websites may use content delivery networks for example to provide you with faster access to contents, or load other data from third party sites that power some of its functionality.
Other uses of third party sites include advertising, by embedding scripts such as Google Adsense or banner ads into the site, social networking buttons and counters, analytic scripts run on third party sites, or the display of other widgets or contents from third party sites.
Most browsers nowadays come with options to visualize those connections right away. In Firefox, you hit F12, switch to Network, and reload the web page that you are on to see all the network connections made to load the website. Here you see all first and third party connections a site makes.
Mozilla Lightbeam, formerly known as Collusion, has recently been upgraded to version 1, a major new version of the add-on for the Firefox web browser (the latest version right now is 1.0.2). The new version comes with performance improvements, as well as better controls over the visualization in the interface.
Tip: Google Chrome users can use Collusion for Chrome to collect similar information in the browser.
It begins to collect data about websites that you visit in Firefox right after installation. The data that it collects include all first and third party connections that are made during visits, and their connections.
That's great for a number of purposes, for instance to find out how websites you like are connected to each other, or how individual companies can track you on the Internet.
You can use it to check up on individual sites to see all connections made when you connect to it, or keep it running for some time to get a clearer picture of the connections between all the sites that you visit regularly.
The default view mode is daily, which you can change to weekly, recent site or last 10 sites.
The top provides you with statistics about your visits, including the number of sites that you have visited, and the third party connection count.
Toggles at the bottom of the screen enable you to hide or display various information, including connections, visited sites, third party sites or cookies.
A click on a server displays additional information in the interface. Here you see the first and last access time, the server location in the world, and the third party connections that were made during connection.
Two alternative visualization modes are available. Clock displays the connections by hour of the day, with visited sites and third party sites depicted as triangles and circles in the interface.
List on the other hand displays all connections in list form for easy access.
If you want to find out how websites track you, or how websites you visit regularly are connected with each other, then you will find that Mozilla Lightbeam is an excellent tool for that task. While I personally would not run it 24/7, it may make sense to run it for a day or browsing session to let it collect enough data that you can then analyze and act upon.