Tom Beddard of subblue created this nifty little Flash application where you can draw your own mathematical butterfly:
Taken from Clifford Pickover's book, Computers and the Imagination, is this experiment that creates butterfly like curves.
The formula is expressed in polar coordinates as:
By changing the A, B, a, b and c parameters you can get some nice results.
It's fun to change the parameters to see what you get: Link - via Cliff Pickover's Reality Carnival