I've used http://buysellads.com http://buysellads.com with mixed results. Lots of sites on there, but mostly with a tech focus. The CPM's and effectiveness of t

I've used http://buysellads.com http://buysellads.com with mixed results. Lots of sites on there, but mostly with a tech focus. The CPM's and effectiveness of the ads vary quite a bit (from crap to very good), so it's worth digging around to find some good cheap ones. And as always, track everything.