Free *regular* application on iPhone - how to display ads?

Question

What ad networks are available to iPhone developers who want to distribute free regular apps (not web applets) that are ad supported?

Prompted by this story on crappy ap

Answer 1

I've seen a Tower Defense game (TapDefense) that makes the user click a link to a paid app in order to save the game. It uses AdMob as its ad distributor.