With increased use of popups on sites - when you enter, when you leave, while you are there - Popup killers (or stoppers) are all rage.
Companies use Popup killers as a marketing tool for frustrated webusers - they kill Popup, webuser uses their service be it as an ISP or Toolbar.
New ISP Disks, Toolbars, and Browsers all now come with some form of Popup killer. While we all want to stop frustration of having several popups being delivered while surfing around web, to webmaster a Popup killer can totally kill a site's normal functionality.
** What do I mean?
It could mean that when a user clicks on your apply button or affiliate link, it won't work! The user can click on it as often as they like. It won't work!