PayPopup

PayPopup.com works well for me because you don't need to click or sell anything. It just gives you an option to use exit popups as well as entry. You can set how much of a delay you'd like for the pop, for instance you might like it to pop up 30 seconds after a visitor has entered your site. You can set the frequency of the popup, either each session or based on a time period. This is a cpm program, and raw hits are counted meaning if a popup loads for the same person 20 times during their visit to your site, you will get paid for the 20 views.
www.paypopup.com