Algorithmic Botany - The definitive resource for L-Systems, this University of Calgary site is maintained by Lindenmayers collaborators. It contains many articles, and a full-length copy of 'The Algorithmic Beauty of Plants'.
Christian Jacob - A collection of papers related to using Mathematica to generate L-Systems.
Fractal Geometry: L-Systems - Introduction to L-systems by Michael Frame at Yale University. Includes background information.
GDesign 1.0 - Generative art tools that create complex 2D and 3D objects breeding LSystems, Cellular Automata, and Shape Grammars. Freeware by Umberto Roncoroni.