7. A method of recommending items to users, the method comprising: providing a user interface that provides functionality for a user to create tags via entry of text strings into tag entry fields of item pages of an electronic data repository, and associate the tags with selected items of the electronic data repository, wherein the tagging system is configured to store user-specific tag data, the user-specific tag data reflecting associations created by the user between the tags and the selected items; outputting for presentation to the user one or more of the tags created by the user; receiving a user selection of a selected tag of the one or more tags output for presentation to the user; and in response to receiving the user selection and without requiring user interaction: identifying at least one of the selected items previously tagged with the selected tag; programmatically accessing item relationship data to identify additional items that are related to the at least one selected item previously tagged by the user; and selecting one or more of the additional items as recommendations for the user; wherein at least said programmatically accessing the item relationship data is performed by a computer system comprising computer hardware.
7. A method of recommending items to users, the method comprising: providing a user interface that provides functionality for a user to create tags via entry of text strings into tag entry fields of item pages of an electronic data repository, and associate the tags with selected items of the electronic data repository, wherein the tagging system is configured to store user-specific tag data, the user-specific tag data reflecting associations created by the user between the tags and the selected items; outputting for presentation to the user one or more of the tags created by the user; receiving a user selection of a selected tag of the one or more tags output for presentation to the user; and in response to receiving the user selection and without requiring user interaction: identifying at least one of the selected items previously tagged with the selected tag; programmatically accessing item relationship data to identify additional items that are related to the at least one selected item previously tagged by the user; and selecting one or more of the additional items as recommendations for the user; wherein at least said programmatically accessing the item relationship data is performed by a computer system comprising computer hardware. 8. The method of claim 7 , wherein the additional items are related to the at least one selected item based at least in part on item-to-item similarity mappings.