I think that would be Firefox or Mozilla.
Some try the Google toolbar, but with all the unpatched exploits we find at http://www.safecenter.net/UMBRELLAWEBV4/ie_unpatched/ it's clear that Microsoft is dragging their feet on this issue. IE 6 has been out for years now.
Bob
What is a Good Pop Up Blocker for Internet Explorer?
Thanks,

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic