1. A caching system for local storage of objects originating from one or more remote origin servers, comprising: a cluster of caching server appliances for retrieving objects requested by clients from a storage hierarchy of sources including dedicated local storage, shared local storage, and remote servers, and for retaining cached copies of selected said objects in said dedicated local storage and said shared local storage; a plurality of dedicated local stores, each associated with a respective said caching server appliance for providing a respective said dedicated local storage to said respective caching server appliance; a shared local store accessible to all said caching server appliances for providing said shared local storage; and a supervisor appliance responsive to all object requests from said clients for determining when a particular object stored in said shared local store should be replaced with another object.