I've used OpenX a few years ago and it worked fine. Now I see that it wasn't maintained for a while hence it became buggy on newer versions of PHP.
Do you know any ad server that is a good alternative to OpenX?
I've used mySimpleAds for a client in the past, and they were happy with it. As the name suggests, though, it may be missing features versus what you were using in OpenX(which was complicated, so...). You'll have to do that for yourself.
Well definitely stay the hell away from openx unless you're shelling out big $$ as part of their microsoft-run enterprise level of service (i.e. its what groupon is on). They gave up years ago on the open source openx, and it shows.
Right now as far as free ad servers that actually are decent is concerned, check out google's doubleclick for publishers (it's the renamed google ad manager). I've been using it on 3 sites for the past 6 months or so and have no complaints so far. https://www.google.com/intl/en_US/dfp/info/welcome.html
I am selling a custom built ad server script. It is much better than any thing else on the market and has been developed by my talented developers for about 3 years. It will not be cheap but you can contact me here for more info: ptoone.com
http://orbitopenadserver.com/ might be a solution. But I never installed that script.