Lightbeam

[Skhilled]

Here's an Addon for Firefox that is useful if you wish to know what 3rd party sites you interact with on the web...and you may not even know it!

https://addons.mozilla.org/en-US/firefox/addon/lightbeam/

After you install it, just surf as usual while keeping an extra tab or window open for Lightbeam. Every time you connect to a site it will visually show you that site and all 3rd party sites that you connect with through that site. Try it, it's very interesting! 

[Skhilled]

Here's a little more info on it:

http://www.theguardian.com/technology/2013/oct/28/mozilla-lightbeam-tracking-privacy-cookies

EDIT: And for Google Chome:

https://chrome.google.com/webstore/detail/collusion-for-chrome/ganlifbpkcplnldliibcbegplfmcfigp?hl=en