4. A computer-implemented method of providing item recommendations to a target user, the method comprising: by a computer system comprising computer hardware, the computer system configured to generate recommendations by at least: by a computer system comprising computer hardware, the computer system configured to generate recommendations by at least: providing a user interface that provides functionality for users of an electronic catalog of items to create and assign arbitrary textual tags to individual items represented in the electronic catalog after selecting said items, such that the textual tags categorize the items represented in the electronic catalog, the user interface comprising a tag entry field on item detail pages of the electronic catalog, the tag entry field enabling the users to create the tags via entry of text strings into the tag entry field; programmatically identifying at least one tagged item that was selected by a target user, the at least one tagged item being represented in the electronic catalog, without receiving a designation from the target user of the at least one tagged item; subsequent to and in response to said identifying, selecting one or more user-defined textual tags associated with the at least one tagged item by at least identifying a threshold number of most popular tags assigned to the at least one tagged item, without any input from the target user, each tag comprising a descriptor selectively associated with the at least one tagged item by one or more users; subsequent to said selecting the one or more user-defined tags, automatically searching the electronic catalog using the one or more tags as one or more keywords to identify a corresponding set of related items; selecting at least a portion of the set of related items to provide as the recommendations to the target user; and outputting the recommendations with associated textual reasons for recommending the items.
4. A computer-implemented method of providing item recommendations to a target user, the method comprising: by a computer system comprising computer hardware, the computer system configured to generate recommendations by at least: by a computer system comprising computer hardware, the computer system configured to generate recommendations by at least: providing a user interface that provides functionality for users of an electronic catalog of items to create and assign arbitrary textual tags to individual items represented in the electronic catalog after selecting said items, such that the textual tags categorize the items represented in the electronic catalog, the user interface comprising a tag entry field on item detail pages of the electronic catalog, the tag entry field enabling the users to create the tags via entry of text strings into the tag entry field; programmatically identifying at least one tagged item that was selected by a target user, the at least one tagged item being represented in the electronic catalog, without receiving a designation from the target user of the at least one tagged item; subsequent to and in response to said identifying, selecting one or more user-defined textual tags associated with the at least one tagged item by at least identifying a threshold number of most popular tags assigned to the at least one tagged item, without any input from the target user, each tag comprising a descriptor selectively associated with the at least one tagged item by one or more users; subsequent to said selecting the one or more user-defined tags, automatically searching the electronic catalog using the one or more tags as one or more keywords to identify a corresponding set of related items; selecting at least a portion of the set of related items to provide as the recommendations to the target user; and outputting the recommendations with associated textual reasons for recommending the items. 8. The method of claim 4 , wherein the set of related items comprises lists of scored items, each list corresponding to one of the at least one tags.