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

You can select how many similar topics to show, and whether to include those from the same forum, at Admin -> Topics -> Topic Settings.

In WSN 9.0+, the display code for similiar topics can be edited via the field manager at Admin Panel -> Topics -> Topic Fields -> Similar Topics. In 8.0 and earlier, the display code for showing similar topics is in your topic 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 topic (including toplists) if desired.