Project Wonderful's ad network became effectively a banner exchange for most people, where money changed hands occasionally and a handful of people were able to make a living. Now that they've stopped operating, I think it's time to bring back the free banner exchange concept (like the original https://en.wikipedia.org/wiki/LinkExchange), but without a central authority.
@jamey A tangent, but: So much LinkExchange history there I didn't know! They received $3 M (in 1997 dollars!) and acquired a shopping cart and sold to Microsoft 18 months later?