Back when I was in the need for an ad server, I remember using phpAdsNew, now called OpenX I believe. It's not for everyone, so I recommend you give it a spin

Back when I was in the need for an ad server, I remember using phpAdsNew, now called OpenX I believe. It's not for everyone, so I recommend you give it a spin before deciding anything, but it does offer a lot of configurability and analytical tools.