1. A computer-implemented method of discovering behavior-based relationships between items represented in an electronic catalog, the method comprising: recording accesses by users to item detail pages of the electronic catalog, each item detail page corresponding to a respective catalog item, said accesses recorded without requiring the users to rate or purchase the items and without requiring the users to create lists of the items; storing a plurality of session records in computer storage, each session record being specific to a respective browsing session of a user and identifying a plurality of items for which the user accessed a corresponding item detail page during the browsing session, at least some of said session records corresponding to different users than other session records; generating data values representing strengths of relationships between particular ones of said items, wherein generating the data values comprises, for a pair of items consisting of a first item and a second item, determining, based on the plurality of session records, at least (1) the number of browsing sessions in which an item detail page for the first item was accessed, (2) the number of browsing sessions in which an item detail page for the second item was accessed, and (3) the number of browsing sessions in which item detail pages for both the first item and the second item were accessed; and generating a mapping of items to related items based at least partly on said data values; said method performed by a computing system that comprises one or more computers, wherein the method enables machine-based discovery of behavior-based item relationships without requiring the users to rate or purchase the items and without requiring the users to create lists of the items.