I know this has been discussed before but there are many new members here i would like to enlighten for the protection of their privacy. here it is again.
Banner ad tracking ... See how it is done. A demonstration has been set up with 27 web sites in a fictional ad network. Visit several of the sites, register at one, and see how your activity is tracked across several web sites, and your e-mail, and then linked to your identity.
After you see what can be done with cookies and if it matters to you, go to go to:
Reveal Web Bugs
Find out who's tracking you on the Web.
You'll get the surprise of your life once you start using it.