Java ODE library
This is a small library that implements a set of efficient Ordinary Differential Equation solvers in Java. You will find the API documentation here and a jar bundle of the classes, as well as a simple example of use, the chaotic Lorenz attractor (you may need to configure your Java plugin).
jPicEdt for LaTeX
jPicEdt is an open source, multi-platform and interactive picture editor for LaTeX and PSTrick which I initially decided to write as a way to learn the java language. It has now become a fully-fledged tool used by thousands of LaTeX users. The project has now been taken over by Vincent Belaiche. Other collaborators on this project are Vincent Guirardel, Jobst Hoffmann and Mathieu Perez.
You can find more on jPicEdt here.