phpadsnew is probably one of the most feature rich programs out there and its free. I have used it, there is a learning curve to it but you can't beat the features. If you need something simple then I'd search hotscripts.com for something they have a bunch there you will find something that fits your needs.
PHPads new has everything in it including the proverbial kitchen sink I think. heck of a program, but heck of a learning curve if you're not familiar with advertising concpets and lingo.
Are you looking for a simple ad rotation system? Or something designed for tracking also? If just rotation - look at Dynamic Drive for an image rotator and some slick ways of serving ads that are more than just a banner rotation. www.dynamicdrive.com/
If you use Dreamweaver look at the Kaos Weaver addin - it's a real simple, easy to use image/text rotater. If you're lookng for tracking along with the rotation - hotscripts has quite a few in several formats.