Similar Entries 2 (6.2) - http://drupal.org/project/similar - doesn't work. But version 1 works like a charm. Version 2 spits up an error on array_filter. Looking at the code, my guess is it's related to some new Views plugin code in version 2.

Related Block - http://drupal.org/project/related_block - would have been cool too. It's much like Similar Entries. Unfortunately, the search algorithm is way too narrow. At first, I couldn't tell if the module was even working. Then by weening the search terms down to 1 (which means it figures out a single relevant term and then searches for only that) I saw some results, but not on many nodes still.

Relevant Content - http://drupal.org/project/relevant_content - is nice in theory. Currently, it's broken. It's in the middle of a rewrite, but it's looking more and more unlikely that it will ever get rewritten.

Related links - http://drupal.org/project/relatedlinks - is not what it seems. It just finds any referenced links in the content and groups them together. Nothing external.

Other modules are term (taxonomy) based. But ideally, you don't need to specify all the relevant terms, and you don't need to explicitly say that two terms are related. There should be a more intelligent way.