Pop-Up No-No! runs in the system tray and automatically blocks pop-ups and pop-unders in Internet Explorer, Netscape, Mozilla, and Opera! It will also block flash ads, frame ads, gif animations, background sounds, and more in IE!

Blocks pop-ups and pop-under ads
Blocks Flash animation ads
Blocks those new Messenger Service ads. (That's those grey, "blackmail" ads)
Blocks non-browser ads spawned by other programs such as MS Outlook.
Supports multiple users!
Quick Internet search.