If you go and look on sites like Freshmeat.net or Hotscripts.com, you'll find tons and tons (and tons) of ad serving scripts. Most of them are just a small rotation script with no real tracking built in to its system. One of the more popular and well-written set of scripts that are out there is phpAds, a system written by the guys over on phpwizard.net, and DevShed.com has a new article telling you all about how to use it and how to set it up on your site.

Of course, they give you all of the links and all of the details on how to get it running on your site, as well as getting your "campaigns" up and running. One of the cool things about these kinds of "banner management programs" is that they allow the clients to log in and take a look at how well their banners are doing (always a good thing). There's even pretty little graphs and numbers in tables for those marketing droids that can only read that sort of thing.