Now, I dont remember if I told you guys about this or not.
www.qscomputing.net Has an Ad Network, that you can simply, upload your banner too.
It doesn't cost a cent, and its very useful.
You can have the adnetwork on your site, IF you want to.
I highly recommened it.
So in other words it is just a banner exchange!
A good one.
If you ask me...