My favorite forum has ads on every page. One header and one footer. Text only as a link to the site or product being advertised. The advertisers pay the site owner himself.

I've bought things from those ads because they're targeting the demographic on that site, not targeting me specifically. They're actually more relevant.

Now that's not probably sustainable, but I have to imagine that the roi for the advertisers is higher than general targeted ads. I've never even clicked on one of those except by accident.