AdMoolah has put together a useful list of links if you’re looking for sites that allow you to contribute content in return for displaying AdSense ads with your publisher code imbedded into them. Probably the most well known of these is Digital Point’s Forums which all participants can submit their AdSense publisher code to. The more you post the more chance you have of earning. Of course forums are notoriously hard to monetize - but I know of a couple of bloggers who make a few dollars here and there this way.