Pop Unders are simple windows that open up when people visit your site BUT are displayed underneath your sites browser window.
This window will open unobtrusively, compared to the likes of pop over, in a completely separate browser and require no modifications to your sites design. It allows you to capitalize on your sites audience without the risk of them not reading YOUR own website content (which can happen with pop-overs)!
NOTE: With Microsoft FrontPage and some other web design programs, when you paste the script into your html, it changes some of the script automatically and it won't work properly. I recommend typing the script in manually if you're having problems.
Here is the script, DON'T FORGET TO REPLACE THE XXXX WITH YOUR CLICKBANK ID!: