Advert Pop Up

Hi,
does anyone have the code to place an advert pop up on a website of the type that many large companies have. They just seem to appear without cause and then you can close them. Do they work by rollover or some such means? Any help would be much appreciated.
Tom

Popup blockers are update very often which means a lot of the big companies change their code often too.
You may find a few that work by Googling for: "interstitial javascript code" or something similar.

Googling for: "interstitial javascript code" or something similar. ?