A WebmasterWorld thread suggests that Google is getting better at either determining or showing that they know which sites are related to others in the related command. The related command is triggered when you click on the "similar" link in the search results, here is a picture:
When you click that link, it basically triggers an advanced query for related sites. The query is [related:www.seroundtable.com. And if you look, the results are much more on target then in the past. Here are sites Google suggests are related to this one:
And there are more and most of them I would consider to be related to this site. In the past, the related command was somewhat of a joke, now it seems pretty accurate.
WebmasterWorld administrator agreed saying:
Totally agree - it's been a long time coming, but related sites are now much more related in topic, rather than some spastic collection of loosely interlinked sites in an odd cluster that does not give a user any value.
Good to see this improvement from Google.
Forum discussion at WebmasterWorld.