In the past I used AdsDaq (which later merged with another company to form ContextWeb). Instead of being CPC, you could set a desired CPM for each unit, and they would match advertisers who bid that much with your ad. If no one wanted to pay that much for an impression, the system served up a back-up ad (like AdSense or an affiliate banner).
It's an interesting and different take on advertising, and it gives the publisher a little more power in the relationship (where-as you have no control over CPC / CPM with AdSense). However, they have since merged with another company to create "PulsePoint." I haven't used them since that merger, so I can't vouch for their current reputation.