Valueclick

One of the largest ad networks with 13,500 sites for display advertising. They do a revenue sharing model where they share 50% with their publishers. Like other large ad networks, they can run media on a CPM, CPC or CPA but since it all runs on the same network, if you do a performance deal with them, volume can fluctuate significantly.

They also do lead generation using a co-registration program. They have a separate network for this, comprised of 1,500 sites.