ObPlug: get the book to get started with the source
This is my obligatory plug for the JMX book:
JMX: Managing J2EE with Java Management Extensions (http://www.amazon.com/exec/obidos/ASIN/0672322889/104-6670791-7933546)
The codebase for the JBossMX project is in many parts based on the ideas, designs and examples discussed in the above-mentioned book. To support the continued development of an Open Source project, please consider purchasing a copy.
An open, cross-platform journaling program.
A scientific plotting package.