I think it'd be useful to have an overview of the various content recommendation modules in this group, and eventually a comparison review in the Handbook.
Warning: several of the modules listed below are under-maintained or abandoned. I've marked the more actively maintained ones (as of 01/2013). If it's not marked maintained, it's probably not.
Featured Content (Maintained)
Very easy, one form, interface to create featured and related content blocks of various kinds. Can create them based on manually chosen nodes or by using simple filters (e.g. all the nodes that have content type "page" and taxonomy term "SEO"). Documentation is built into the form page, so the user can just start building the blocks without reading external documentation ahead of time. Currently, the Featured Content module has very high reviews on drupalmodules.com and is in the top 5 modules in terms of ratings.
Similar by terms (Maintained)
I've used this and like it a lot. It only provides a block, no views integration yet, but in general it's provided very, very good matches without a major performance hit. Drupal 6 version has some bugs as of December '08.
Relevant content (Moderately maintained)
Very, erm, similar to similar by terms - but it uses a different algorithm, has some different options for block creation and display, and the Drupal 6 version appears to be better supported than similarbyterms.
Similar nodes (abandoned)
Very similar to "similar by terms" but similarnodes requires views - see issue here for potential merging: http://drupal.org/node/255183
Also, doesn't currently work - http://drupal.org/node/293503
Also, no sign of a D6 version.
Node Relativity (maintained)
For connecting nodes in parent-child relationships (in D5 and D6).
The Node relativity access control module defines user roles (for D5 only, for now). An experimental patch adds access control for D6, but needs more work.
The Node relativity 3.0 module is a complete rewrite for D6 with much more functionality (but stalled?).
Node hierarchy (Maintained)
Node hierarchy is another module that allows nodes to be children of other nodes creating a tree-like hierarchy of content (in D5 and D6). Only users with editing rights can add children to a node.
Related Content (Moderately maintained)
Documentation in the handbook. Relies on Views. Provides a tab on the node view.
Requires users to pick the related nodes, but can use views to suggest them.
Provides an API which could be used to make this process more automated.
Related Links (maintained)
http://drupal.org/project/relatedlinks D5, port to 6 underway?
According to the project:
* Parsed links: links that are retrieved from the body of a node.
* Manual links: links that are added manually.
* Discovered links: links that are discovered by the module using various criteria, including the category terms of a node and suggestions provided by the search module (when enabled).
This is yet another module that allows you to create relationships between nodes (inD6). It requires editing rights for this.
By the same author as CRE. Intends to provide a framework for similarity rules.
This module allows the administrator to create a wide ranging set of similarity calculations. It can out of the box, calculate similar nodes of a specific type using the search index, taxonomy terms and node title. These similarities are calculated on cron run.
All these similarities are then exposed to Views. Using the provided argument, you can create a listing of similar items for a node. For instance, you can create a View page at node/%/similar. That page will use the Similarity argument (Similarity NID).
Content recommendation engine (abandoned)
Similar entries (abandoned)
Drupal 5.x & Drupal 6.x versions. Uses title and body fields, MySQL only. Not terms/taxonomy. Scans title and body fields and automatically generates a list of 'similar entries' based on an algorithm that apparently is used elsewhere. Seems to generate pretty good, but not always perfect matches. Works 'out of the box' - no configuration required. Outputs to a block. Good way to generate 'related pages' type list for nodes.
User related content (Drupal 5, i.e. abandoned)
node -> user relationships
Matches conversation topics around keywords etc. (being trialled on drupal.org)
Future development planned to provide a generic Recommender API module for D6
Integration with the sphere.com web service - does both internal and external related content links
Related Block (abandoned)
Recommended Posts (abandoned)
Associated nodes (abandoned)
Taxonomy and CCK based weight/scoring of relationships.
More like this (abandoned)
Hot off the RSS Feed of new modules.
Node Recommendation (minimally maintained)
Drupal 5.x & Drupal 6.x versions. Works off terms if using taxonomy, but also allows manual selection of nodes for individual pages with or without taxonomy. Outputs 'Related Pages' in a block. Good option for someone with not too much technical knowhow.
Editor or privileged user defined relationships - [author edit: not abandoned, demonstration code, development to begin soon: Its real purpose is to pull in recommendations from other modules listed here and make it very easy for people to select what they think are the related content, so I'm very interested in a common API...]
Does a tag cloud of terms on taxonomy/term pages - depending on their frequency of association together on nodes. Nice idea, no release yet.
Incorporate the classical user-user and item-item recommender algorithms. Aiming to be a generic algorithm engine for all other recommender modules. This is the D6 port to http://drupal.org/project/pivots.
Abandoned in favour of Similar by terms
Keywords (to help make this page easier to find): related pages, related nodes, related posts, similar pages, similar nodes, similar posts