I'm glad to "officially" release our open-source ad server, Adjector.
It's been around for about a year in a semi-stable state; ad serving worked fine but there was almost no UI to speak of and click tracking was super broken. I've removed our attempt at a GUI using GWT and written a nice simple HTML one; and fixed form validation, click tracking string substitution, and a bunch of other bugs. Perhaps most importantly it also finds and displays an appropriate random ad three times faster, including view tracking (from about 55 ms to 15 ms). This is due to the weighted probabilities being pre-cached with the new design.
My goal with Adjector was to create a fast, slim, flexible, open-source ad server, which AFAIK didn't exist before. There's quite a few other open source ad servers, most based on phpAds. But PHP doesn't make for a great anything server, and none of the other servers that I know of can integrate directly into your application and allow the client to only make one request, much less allow your application to share the same database and connections. So I think I've accomplished my design goals here, Adjector is so fast that you can't even tell it's there, it has all sorts of ways to weight ads, it can display and randomize text ads, Adsense-style, and it is Python and open-source so it's easy to hack on.
So if you need an ad server for your site, and you can get Pylons going, Adjector might just be what you're looking for. And we're definitely looking for more developers, too.