In this paper, we describe the ClusterProbe, an open, flexible, scalable, and Java-based cluster monitoring tool. The tool provides an open environment by developing a multiple-protocol communication interface that can be connected to various types of external accesses from the clients. ClusterProbe is flexible that the monitoring tool can be easily extended to adapt to the resource changes by using the Java-RMI mechanism. In addition, the design of ClusterProbe allows it to scale up to commodious capacity with its cascading hierarchical architecture. Several useful services are implemented based on ClusterProbe, including the visualization of cluster resource information in various form and cluster fault management. The tool has been used to assist the execution of a cluster-based search engine and a distributed N-body application. All experiments demonstrate high efficiency.