Here are a few useful Gimp plugins. They are licensed under the GPL, and have all been updated for Gimp 2.x. Let me know if you need earlier versions. (Or update your version of Gimp!)
LSystem plugin. M. Gomulinski wrote a Gimp v0.99 plugin to render LSystems. Since I could not contact him, I have made the modifications required to use the plugin with Gimp 1.2. It has not yet been updated for 2.x.
Some stuff that was here has been removed, because I haven't had time to work on it for a while. Let me know if you're missing something.
I'm hodsond at ozemail dot com dot au, if you want to email me.