I'm sure you know the feeling.

I installed the Free Edition of Pop-Up Stopper from Panicware, and now I almost never see a pop-up / pop-under ad.

One usage note with Pop-up stopper: to ALLOW a pop-up window, hold the CONTROL key down while clicking on the link. Some websites do use pop-ups for things like displaying tables, event listings, etc....fortunately, Pop-up Stopper notifies you when it's stopping a pop-up, so you'll know if you click on a link and nothing happens, it's because the website wants to show you a pop-up window for the information you've requested. Just hold the Control key down and click the link again.

AN ALTERNATIVE APPROACH:
Download Google's Toolbar -- it's really useful (you can search Google without actually going to Google.com first), AND it blocks pop-ups!