I've had good results with Adcenter in the last few months. My click through rates were only about 4% (my ads suck), but conversion rates varied from 30% to 50% week to week.
My Adwords account burned money with the same ads. I had more traffic, but only converted about 2%.
The ad costs were a little bit lower than Adwords, but I needed conversions, not traffic.
Some of the other posters sound like they are interested in the the MS Publisher netowrk (similar to Adsense). It is still in Beta, but you ccan let them know you are interested here:
Microsoft Publisher Program - Microsoft Digital Advertising Solutions