I am trying to find a good way to serve ads that are purchased by advertisers while mixing them in with Google Adsense. I've never used any external ad servers or anything of that sort, so don't know the general idea of it. But I am working with a vBulletin site and want to optimize the ads by server purchased ads and adsense at the same time to measure CPC/CPM.
If you don't have any system to rotate your ads, you can use OpenX. In case you didn't know. It's free as well, and I'm using it myself.