RPTools is a set of five open source programs written in Java designed to assist with managing roleplaying games. They include
MapTool – An online, multiuser, networked, graphical, interactive, programmable virtual tabletop
TokenTool – Create quick and simple consistently sized tokens for use with any digital battlemat that can use PNG images.
CharacterTool – A configurable tool for managing the number-crunching of building and maintaining player characters.
InitiativeTool – Keep track of the flow of encounters
DiceTool – A general-purpose dice rolling application with programmable buttons and extensive customization.
I’ve seen quite a few open source RPG-related projects, but these are really slick looking and are being very actively developed.