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

You can select how many similar listings to show, and whether to include those from the same category, at Admin -> Listings -> Listing Settings.

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