Have you ever heard of Project Wonderful?
It's not CPC or CPM, you pay for each seconds/day that your ad is shown.
You bid for any project wonderful ad spot, and as long as you have the highest bid, your ad is shown. If your ads aren't showing, then you don't lose money.
http://www.projectwonderful.com/It's worked quite nice for me so far. If you've got a popular website, I'm sure you'll get loads of bids!
