Silk

Try playing with the rotational symmetry.

http://i.imgur.com/IJgi1gc.png

click 1 spot over and over, quickly - you'll get a white line that gets a little thicker as it's fully saturated.

click a different spot over and over, but hold it a little longer, you'll see the delta-t effect tracing the same pattern over and over.

one way to implement such a thing, you'd want a function to look at a pixel and it's nearby neighbors, and brighten. red = red+10 or some such. next you'd want a linedraw function, given x,y,t - pick a spot near x and y, step a few pixels away in some direction, brighten the stuff between. repeat that, with a bias tward the direction you're travelling, and cap it out some how. something like steps=(x+y+t)%100 +10. min length is 10, max is 100, kinda vary with time.

Anyway, that's the basics. you can futz around with the specific functions to give a nice effect.

edit

ah, mynegation's comment is probably the truth. particles with inertia would do just the right thing for biasing direction.

It's using JavaScript (D3) and Canvas. There's an interesting mix of JavaScript libraries included in addition to D3: jQuery, Underscore, Knockout...

This thing is fun.