NetworkTools has 2 components: A protocol analyzer which plays a TCP client or server or pipeline, which sits between a client and server and shows how they communicate. A port scanner can scan a range of IP addresses and ports. See screenshots.
JINX is a Java-based multi-user Unix-like environment that allows each user to remotely run their own java programs. Also the home of CurseHTTPD, a non-blocking java HTTP server.
JTelnet is a simple telnet library for Java with a few demo applications. One is a simple command line telnet, another is a Swing component that includes VT100 emulation.
FreeHost3270 is a complete 3270-to-web solution, providing single-point deployment of legacy 3270 applications to unlimited clients. It delivers either a java applet-based TN3270 emulator, or a stand alone application with rich GUI.