Action Announcer script alerts your visitor that they are about to leave your page. You can customize the screen that appears and try to convince them not to leave. If they purchase your product they never see this page.
Snap.com is a bit of a bust. A cool “link preview” tool, but I was not happy that I could NOT get rid of the advertising (not ability to pay to go away). This tool allows you to preview the website that you are about to click and link and go to that website.
20 Dollar Banners seems to be a very cool site with some customers that you will recognize. You can pay as little as $20 for a banner (as well as look at banner packages) www.20dollarbanner.com