Kind of a plug i suppose, but they do it for free so it's really worth a mention.
It seams like a great way to get relative backlinks without the need to link back. You place a piece of code on your website, and any referrals you make through the banner gives you credit on their network, and these credits are used to gain backlinks automatically, minimal effort really for great relative backlinks.
All backlinks to you are one-way, not reciprocal, and the websites where your links are placed are other members websites, not link farms..
URL: relevant Linking - Website SEO Experiment