Java
Lace		A package for rule-based modeling of images, 3D forms, and music. Lots of demos.
Worm		An applet that displays a wriggling 3D wireframe worm.
Glow Toy		Another eye candy applet. You might prefer Natasha's Lovely Purple Toy instead.
Particles		Drag particles around and watch them interact with each other. Fun. Useless.
More Particles		More fun, fun, fun with particle systems.