I don't know if this helps, but a search for adsense competition (on google no less) brought up the following:
http://www.associateprograms.com/articles/208/1/Adsense-Comp...
Which has a list of different things like adsense you could use. Out of curiousity, is there any reason why you can't switch to using these companies for advertisement? I can see how google has a complete monopoly on ad-words, but surely adsense is a free market.
AdSense needs competition. Badly.