If you've ever played Dopewars, you've wished this game was a multi-player game. Well, now it is, but it's a long way from ever being complete. Along with the basic DopeWars game, the more advanced Chaos Corporation multi-client framework was introduced. This is a framework to support a single server required to handle communications to many clients. Each client need only communicate with that server.
This contains some very nice examples of Client/Server code in java over TCP/IP.