I once saw on the forum a link to some free software that allows you to insert code into your site and then pick what banners go where from a central pool.
The aftermarket support. For example, we do work on Oscommerce based ecommerce sites. Oscommerce is free, and thousands of stores use it. But some need modifying beyond the original remit, and that's what we get paid for
Look on the OpenX forums (fora?) and you'll see people being paid to do things above and beyond what it's coded for