Algorithmic Botany - The definitive resource for L-Systems, this University of Calgary site is maintained by Lindenmayers collaborators. It contains many unique articles, and a full-length copy of 'The Algorithmic Beauty of Plants'.
BioSchematics - An L-System interpreter and drawing tool for linux.
Christian Jacob - A collection of papers related to using Mathematica to generate L-Systems.
New ALife Database - A collection of automatically collected L-System links.