The Ghostery Firefox Addon.
You can get an alert bubble on the upper right side of the browser window upon page loading for every page and see which cookies are being applied to your browser. You can then operate on these cookies (e.g., block).
I noticed how some of the content sites are demonstrating the Outbrain cookie -- this implies that the Outbrain widget is being installed. Nice.
From the Ghostery website:
Ghostery sees the invisible web - tags, web bugs, pixels and
beacons. Ghostery tracks the trackers and gives you a roll-call of the ad
networks, behavioral data providers, web publishers, and other companies
interested in your activity.
After showing you who's tracking
you, Ghostery also gives you a chance to learn more about each company it
identifies. How they describe themselves, a link to their privacy policies, and
a sampling of pages where we've found them are just a click away.
Ghostery allows you to block scripts from companies that
you don't trust, delete local shared objects, and even block images and
iframes. Ghostery puts your web privacy back in your hands.