Clustering provides a viable approach to building scalable Web systems with increased computing power and abundant storage space for data and contents. We present a pure-Java-based parallel Web server with load balancing features for a heterogeneous cluster environment. Our design introduces a special in-memory cache layer, called the global object space (GOS), which is an integration of specially set-aside physical memory of individual participating cluster nodes. The GOS provides a unified view of cluster-wide memory resources and allows transparent accesses to cached objects independent of where they are located physically. The core of the GOS is a mechanism that performs dynamic in-memory caching of frequently requested Web objects. Using a technique known as cooperative caching, a requested Web object can be fetched from a node’s local cache or a peer server’s local cache, with the disk serving only as the last resort. A prototype system based on the W3C’s Jigsaw server has been implemented on a 16-node PC cluster. Three cluster-aware cache replacement algorithms were tested and evaluated. The benchmark results show good speedups with a real-life access log, proving that cooperative caching can have significant positive impacts on the performance of cluster-based parallel Web servers.