12. A system, comprising: a server system that provides user access to an interactive catalog of items, said interactive catalog including item detail pages that correspond to particular catalog items, the item detail pages providing functionality for users to tag individual catalog items via entry of textual strings into tag fields provided on the item detail pages, the item detail pages thereby enabling each user to create one or more tags, and to associate each tag with one or more catalog items; a data repository that persistently stores user-specific tag data specifying the tags and tag-item associations created by the users via the item detail pages; and a recommendation system, said recommendation system comprising a recommendation interface accessible to users via the server system, said recommendation interface providing functionality for a user to view a list of the tags created by the user, and to generate a tag-specific request for recommendations corresponding to a tag selected from said list by the user, the recommendation system capable of responding to the tag-specific request for recommendations by at least: (a) programmatically using the user-specific tag data to identify a plurality of catalog items that have been tagged by the user with the selected tag, (b) without requiring further user interaction, programmatically accessing item relationship data to identify a set of additional related items that are related to the catalog items tagged by the user; and (c) selecting tag-specific recommendations from the set of additional items for output to the user; wherein the server system and the recommendation system are implemented by one or more computing devices comprising computer hardware.
12. A system, comprising: a server system that provides user access to an interactive catalog of items, said interactive catalog including item detail pages that correspond to particular catalog items, the item detail pages providing functionality for users to tag individual catalog items via entry of textual strings into tag fields provided on the item detail pages, the item detail pages thereby enabling each user to create one or more tags, and to associate each tag with one or more catalog items; a data repository that persistently stores user-specific tag data specifying the tags and tag-item associations created by the users via the item detail pages; and a recommendation system, said recommendation system comprising a recommendation interface accessible to users via the server system, said recommendation interface providing functionality for a user to view a list of the tags created by the user, and to generate a tag-specific request for recommendations corresponding to a tag selected from said list by the user, the recommendation system capable of responding to the tag-specific request for recommendations by at least: (a) programmatically using the user-specific tag data to identify a plurality of catalog items that have been tagged by the user with the selected tag, (b) without requiring further user interaction, programmatically accessing item relationship data to identify a set of additional related items that are related to the catalog items tagged by the user; and (c) selecting tag-specific recommendations from the set of additional items for output to the user; wherein the server system and the recommendation system are implemented by one or more computing devices comprising computer hardware. 16. The system of claim 12 , wherein the server system and the recommendation system are implemented by one or more machines.