1. A computer-implemented method, comprising: receiving a data request from a client via a service interface, said data request identifying at least a first item, a first event type, and a second event type, said first and second event types corresponding to respective types of user events that can be performed with respect to particular items; and responding to the data request by looking up 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.