Blocks onbeforeunload
With popup blockers you can block those annoying popups that some pages insist on displaying when you first visit a web page or when you try to leave. Some web sites have now taken to trying to block you from leaving their page by using an in page overlay that pops into the page when you try to leave to ask you to reconsider. These exit blockers are just about as annoying as the popups that they have replaced.
Read More http://javascript.about.com/library/blexitblock.htm
