I was just looking into ad servers to start monetizing my blog. I've found the following (haven't tried myself yet):
All these are full-fledged ad servers where you can target ads by geographical location, etc. Also at least one or two of them let your advertisers have separate accounts where they can see the performance of their ads.
Furthermore, AdSpeed and BittAds both have a Wordpress plugin.
Now, one thing to keep in mind is that they do all ad targeting and serving magic on their servers. So most cost a few dollars a month (OpenX and BittAds are free though, if you don't exceed a certain number of impressions). You can also install an ad server like OpenX on your own hosting, but from what I hear it can use quite a bit of resources.