Perlin noise generates a field of vectors, that particles with a small physics engine can interact with. This site is a customizable version of the background on the home page.
Options
Max Particles
Initial Particle Count
x,y Offset
z Offset
Random Particle Force
Random Particle Count
Flow Field Magnitude
Drag Force
Trail Length
Trail Segments
Trail Hue
Trail Saturation
Trail Lightness
Show vectors
Mouse Gravity
Particle Edge Mode