Open Ads, which was previously called PhpAdsNew, is probably the most popular open source adserver available. I've been using it for years, though I've probably only used a fraction of its features.
I've decided to start using it to serve ads on some of the blogs I run (I'm already using it to run ads on other sites).
Ease of use primarily.
I like the idea of creating ad zones and not having to think about them again !