Particle-test-room is a project mainly written in Java, it's free.
three.js-gwt particle test
This is three.js Particle demo by GWT http://www.akjava.com/demo/particle/
based on http://mrdoob.github.com/three.js/examples/webgl_particles_random.html http://aerotwist.com/lab/creating-particles-with-three-js/