Go to Yahoo and type free pop up eliminator in the search. This is the one I have. It works great. You have to turn it off when you do karma and smilies and shit though.
Make sure you leave the "sound alert" mode on. (I especially like the "BOING!!!")
It's a beautiful feeling everytime you know one of those motherfucking ads has been "blocked"
Now KaZaa has this fucked up ad that takes over your sound card and a voice comes over (quite loudly, too) and says "Click here if you want..." Fucking marketing spammers.