Ad Placement and Management

This is one area which my site is very weak on.

I know very little about how it works or how to implement it. I've spent countless hours looking through all the Ad Management Sites and nothing seems to be worth investing in.

What I want is an Ad Management system that will provide me with information on the number of exposures per month that each affiliate is allowed, the cost and ROI of each ad placed, and information on the affiliate's renewal. It also needs to be an ASP or PHP solution.

Anyone have suggestions about good (or bad) Ad Managers that they've used?

FYI: I'm not talking about PPC Ads...I've tried them and removed them all from my site within days of their inception.