Hi JD.
OpenX is great and it's free. Like Wordpress, you can either run it from your own server or they have a hosted version. And a lot of adult webmasters use it, so it's easy to find someone that can answer questions and help you out.
Another option is
Smart Spots from the same guys that make Smart Thumbs. It's not free ($50). But it might have some additional features or better support than you would get with OpenX.