Not Flamepainter or iSketch.
I found it on Stumbleupon maybe around 2010-ish. You could change how many trails would be made and pick different ways for the line to look/taper, possibly including colors, but one was definitely rainbow. The trails didn't originate perfectly from your cursor, you sort of 'flung' them around with physics that tracked toward your cursor.
The interface was very plain and clean, you'd just click which trail you wanted and how many axes of symmetry to mirror around, then drag away.
It was not a game or image creation program at all, just a pointless web toy. It was fun and mesmerizing!
