Developer website: All Too Flat Software
Particles is a screen saver of particles combining and exploding as they bounce around the screen. The parameters of the physics model are highly customizable, and include the following interactions: gravity between particles, fusion of colliding particles, elastic collisions between particles, and much more. Supports multiple monitors.
Version 0.7b adds customizable particle colors, save/load parameter settings, and fixed a bug when previewing in OS X 10.3.
Mac OS X 10.1 or later