Advertising networks that pay per click, but are flexible.
mrtron: I have found that if you do your homework and use well targeted ads you will make much more using non-PPC advertising. 
edit: I was avoiding introducing too many new acronyms, but I was suggesting CPALet's say you use some sort of program and get paid per lead - I would suggest CJ/Linkshare/Neverblueads[1].  Then track the ads well, see which work and fit in with your site.  Look for a clickthrough rate of at least 0.10% (although depending on how you display/cycle the ads this could be off).  Then from there look to have a conversion rate of definitely better than 1/10, aim for about 1/3.If you work in those numbers (probably your first attempt will be about half that), you will make more than PPC.  You will also completely control how you serve ads and not be bound by the conditions of something like adsense.I used to work for a major website, and they tended to make more per click on their ad campaigns that were revenue sharing than PPC as well.Because of how behind the times the ad industry is in terms of AJAX sites, I really doubt you will find someone who allows you to change the ads in the method you desire.[1]http://www.neverblueads.com/signup?ref=aff_66639 if you want to signup (that would give me a referral).  It is pretty highly rated.