I've heard the ad exchange/trade/etc before, but I'm curious how others actually implement this. Do you make an iAd-esque banner and advertise your other apps?

I've heard the ad exchange/trade/etc before, but I'm curious how others actually implement this. Do you make an iAd-esque banner and advertise your other apps? Do you use an existing system/framework? If its your own implementation, how do you manage/track impressions?