At Admin Panel -> Settings -> Switches, the "similar articles" switch activates a list of articles related to the one being viewed, on the article details page. The matches are automatically calculated by MySQL based on the article title. Note that this requires a MyISAM table format, or MySQL 5.6 for InnoDB.

You can select how many similar articles to show, and whether to include those from the same category, at Admin -> Articles -> Article Settings.

In WSN 9.0+, the display code for similiar articles can be edited via the field manager at Admin Panel -> Articles -> Article Fields -> Similar Articles. In 8.0 and earlier, the display code for showing similar articles is in your article details template at Admin -> Themes -> Manage Templates (search for 'BEGIN SIMILAR' in there) and can be copied to anywhere else within the scope of a article (including toplists) if desired.