Adblock

This is the only one I've actually bothered to check out but I love it. You can block nearly any ad, even some that other programs (like Admuncher) routinely missed.

Right click on the ad and choose "Adblock image" or "Adblock iframe". You'll be given a dialogue with the ad's URL. You can use *'s to basically block a whole ad-spewing domain at a time too, which is why it's so awesome.