We investigate how to improve Web cluster performance through cooperation protocols for a shared use of the main memories of the server nodes (global caching). Many popular schemes to locate requested documents are based upon locality information sharing (namely, Informed cooperation); we also propose a novel approach for document lookup through a query broadcasting mechanism (namely, On-demand cooperation). Moreover, we consider the Migration and the Handoff mechanisms to retrieve successfully located documents. The experimental results show that, in a static Web scenario, the On-demand with Hand-off scheme performs similarly to the Informed with Hand-off scheme, whereas the On-demand with Migration scheme outperforms the Informed with Migration scheme. On the other hand, in a scenario with some dynamic requests, the On-demand cooperation experiences a significant performance degradation with respect to the Informed cooperation.