AdsGone is good as well. Since keeping up with the popups can be a pain in the butt you can also add sites you come across that popup, real useful when you try to watch a mov you just downloaded and it sends you to an unwanted website. Just throw the site into AdsGone and you'll never see it again. I have approximately 2500 sites that are blocked, and my statistics show that from when I first installed it to now (about 3 months ago), its blocked 16,000 ads!

I also use Ad-aware which is great too.

AdsGone: http://www.adsgone.com