Currently, there is an incomplete related content modules comparison in the Organizing content with taxonomy section. There needs to be a broader comparison added here that cover modules that do not just focus on taxonomy.
The discussion of similar content recommendation modules is happening at on the Taxonomy Drupal group.
Check out the Featured Content module to create featured and related content blocks. Here's the project summary.
Similar / Relevant by term is based on the taxonomy terms assigned to content. Blocks are available based on similarity within each of the defined vocabularies for a site as well as a block for similarity within all vocabularies