So how about a search engine where, if you pay for it, it ranks sites without ads at the top. And if you click a link to an ad-less site, that site gets some of the money you paid to the search engine.
Maybe combine it with a social network that works a similar way, though that's harder to do because you don't control what people share.
The major downside is that there would be a central place (the search engine) where the money gets distributed. It would be better to have something non-centralised. Still, I might use a search engine like that.