I recently noticed that eigenclass.org lists the top referrers to each page (look at the bottom of the page). It strikes me as an excellent idea from a SEO standpoint as you automatically create reciprocal links to pages. The pages listed will probably both have contents related to the topic and be popular (i.e. probably be useful). It also creates an incentive for sites to link to you.

That leaves the question why there aren’t more sites doing it. Personally I think it’s simply something that people haven’t considered. It’s not awfully hard to do.

One risk is that spammers will catch on and spam the server with referrals from the sites that they want to list (e.g. in addition to posting spam comments). It might however be easier to discriminate between spam and legitimate referrals than between e.g. spam and legitimate comments. The spammers has to commit several bots to outrank legitimate links (assuming only unique IPs are counted), and will probably not have a natural-looking temporal distribution unless the spammer spreads the referrals out over a long period of time. Additionally there’s no need to display the links instantaneously, one can wait for e.g. a week before deciding whether the referrals are legitimate. One could even scrape the referring url and run it through a bunch of checks (checking whether the contents is sane, what search engines think of it, whois age etc) since the number of urls/domains to validate will typically be small.

In any case this is certainly something that I’m going to keep in mind and apply when I get the opportunity.