A pop-up blocker (sometimes called a pop-up killer) is a program that prevents pop-ups from displaying in a user's Web browser. Pop-up blockers work in a number of ways: some close the window before it appears, some disable the command that calls the pop-up, and some alter the window's source HTML. One problem with pop-up blockers has been that they cannot always differentiate between an unwanted pop-up window and one that is user-requested. Programs that can consistently do so are sometimes referred to as intelligent pop-up blockers.
To circumvent pop-up blockers, some frustrated marketers are turning to products that detect their presence and convert pop-ups to other formats, such as floating ads (which appear over a user-requested Web page and display for a specified time period before disappearing again). According to Joe Apprendi of Falk eSolutions, whose company offers such a product, up to 30% of paid ad impressions are blocked by browser software. Users may not appreciate the floating ad any more than they did the pop-up version, but they aren't likely to have a program installed to prevent its display.
Free pop-up blockers, such as STOPzilla and Panicware, are readily available for download. Both the Google and Yahoo toolbars include the ability to block pop-ups; that capacity is also enabled by default in Microsoft's XP Service Pack Two (SP2) settings for Internet Explorer.