I would recommend OpenX. It is an open source ad management system that you can download and run on your server. I have only been using it for a little while, but there are tons of features. I am running it with wordpress and have had no problems. It does have a bit of a learning curve, but if you work through it, (I'm still a newbie for sure) I can see that it has some outstanding features. It has good stats, and the ability for advertisers on your site to directly control their ads. Lots of cool stuff.
www.openx.com