Link exchange comes round again - Social, Metered, No-Cost Ad Exchange: LinkLike

Social, Metered, No-Cost Ad Exchange: LinkLike: "Blogger? Publisher? Startup? LinkLike can help you increase unique visitors. Its social features help you find like-minded site owners. Its metered ad service gives you a useful way to exchange ads with your partners. 'Metered' means that for every user you send to their site, they must send one to you. Oh, and it's free. "