The Common Open Research Emulator (CORE) is a tool for emulating networks on one or more machines. You can connect these emulated networks to live networks. CORE consists of a GUI for drawing topologies of lightweight virtual machines, and Python modules for scripting network emulation.
Bean Bowl is the a fun and useful Java lab, allowing you to play around with Java classes and objects using direct manipulation techniques instead of writing long main() methods. Use it either as an application or as a GUI component framework for you own