Jython is a Java implementation of the Python scripting language. (Just to confuse matters, it used to be the JPython project).
I am primarily interested in using Jython in conjunction with VTK. This provides yet another scripting option for VTK and, assuming you're using Java Swing, it allows you to easily script user interfaces too.
For more info on using Swing in JPython, check out FermiPython from Jerry Guglielmo. (download page)