Debugging is an essential part of parallel processing. However, developing parallel debugger is difficult. This is especially true for cluster computing where heterogeneity presents. In this paper, we present a scalable distributed debugger for cluster computing. First, a client-server debugging model is introduced. Then the design and implementation of a practical distributed debugging system based on this model are described. Finally, the debugger is compared with related works. The newly developed distributed debugger is based on the sequential debugger gdb and dbx, and can efficiently support debugging various MPI and PVM programs. It has the capability of controlling the execution of up to 256 processes. Its interfaces are completely implemented by Java, and its graphical user interface is the same on all computing platforms. Therefore, it basically meets the three general goals of High Performance Debugging Standard concerning parallel and distributed debuggers. In addition, it is portable, easy to learn and easy to use.