There is a beautifull L-system and GD sample code on the Issue 9, Spring 1998 (Volume 3, Issue 1) of TPJ, by Jason Reed.

The text of the article is not available at TPJ, but you can find it here. There is also this page with some realy impressive L-systems, perl code, pictures and rules included.

And if you want to set the bar realy high, you could check disney meets darwin e pro-actively interactive evolution for computer animation.

Think OpenGL or VRML for eye-candy :-)