[MrEyes]

I am not aware of an addon for this but I do something similar on my site using Google Ad Manager (free)

In GAM you define your ad locations and then add a small code snippet to your templates, this is a one time job.

When an advertiser comes along, you add their details to GAM (i.e. max impressions, date ranges, banner images etc etc). Once you authorise it GAM will automatically serve the banners to your site based on the contract you define. In addition to this, as an option, the advertiser can be invited to use GAM to view the progress of their banner campaign (i.e. how many impressions/clicks etc)

GAM does not allow a customer to upload their own banners you have to do this yourself, which IMHO is a good thing otherwise a customer may upload something that you don't want on your site or breaks the site. Nor does it include any payment systems, you have to do this yourself.

One thing I like about GAM is that you can define "default" banners, so if you have no customers for a particular ad slot your default is shown. This could be an "advertise with us" image or Adsense banners.