ArticleTrader
probably has one of the best offers out there. They will display your Google Adsense code 50% of the time 1) on all your articles, 2) every time someone visits their site through your custom link, and 3) anytime someone visits an article through the RSS feeds you incorporate into your own Websites.

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Plus increase your traffic with their Adsense ad-exchange tool. The tool works by showing your ads in place of the Public Service Announcements (PSAs) in your Adsense. (Kind of like a banner exchange but specifically created to replace the Google Adsense PSAs the PSAs pay you nothing and show up when Google has nothing else to show.) If you are not using the alternate ads at the moment (the google_alternate_ad line of adsense), this Banner Exchange is an excellent way to improve your traffic for free.

Ad Exchange