1. A computer-implemented method, comprising: providing a service interface that is configured to enable clients to generate requests for item recommendations, said interface providing a recommendation request format that enables the clients to limit the requests for recommendations based on event types, each event type corresponding to a different type of user behavior; receiving a request for recommendations from a client via the service interface, said client being an interactive computer system that is accessed by users, said request for recommendations identifying at least a first item, a first event type, and a second event type, said first and second event types corresponding to different respective types of user events that can be performed by users with respect to particular items; and responding to the request for recommendations, by execution of computer code by one or more processors, by looking up from a database, and returning, a list of associated items that have an association with the first item, said associations being based on a tendency of users who perform the first type of event on the first item to also perform the second type of event on another item, wherein looking up the list of associated items comprises looking up the associated items from a dataset that corresponds to said first and second event types.