While searching Google Groups today I noticed some “Related Pages” links appearing as part of an AdSense ad block, this displayed a section of sponsored links with related pages listed underneath, as pictured below.
Image on left is from Google Groups and right-hand-side is from GMail
It looks like Google are currently testing related content as they have recently made changes to their robots.txt file, disallowing access to the /relcontent folder. In addition to Google Groups I’ve also noticed related pages appearing in GMail, this was actually first spotted in June 2004 but I’m not sure if it’s been active since then, I don’t use GMail too much!
I would assume this won’t become available as a new AdSense ad format because they’re not paid ads and Google will just use this within it’s own network of sites. It’s unclear at the moment how these links are chosen, some seem to be generated from recent news stories as many of the listings are dated and from within the last few weeks, while other sites are selected somehow as related to the page content, most likely via an organic search algorithm.