NoAds

NoAds is a completely free security companion developed by SouthBayPC that stops pop-up web ads. This program allows you to visit web pages without annoying ads interrupting your session. It is easy to use and is fully configurable, empowering you to specify which ads to stop.

Moreover, it works while staying hidden in your system tray. NoAds supports Internet Explorer, Firefox, Netscape, and AOL. Note, however, that it can't stop messenger service ads as a different program from the same developer is devoted to disrupting it called Stop Messenger Service Ads.