1. A method of adding items and predefined item categories to a recommendation service that uses collaborative filtering to recommend items to users from a set of service items within a set of service categories, comprising: providing users an option to rate items, including non-service-items and items in non-service-categories; when a user rates a non-service-item that falls within a service category, adding the non-service-item to the service; when a user rates a non-service-item that does not fall within any service category, recording the item and associated user rating in a repository without adding the item to the service; and when a quantity of rated items recorded within the repository for a non-service-category reaches a level that represents a desired degree of reliability for recommending items within that category, adding the category and associated rated items to the service.