The standard map is the first chaotic thing I remember seeing, probably in an AfterDark screen saver. It has an interesting physical interpretation involved rotors, but that isn't important for the pretty pictures.

$$ x_{n+1} = x_n + K \sin(y_n) \mod 2\pi $$

$$ y_{n+1} = y_n + x_n + K \sin(y_n) \mod 2\pi $$