I already have PD Particles - which is a very cool little proggie! I created a tutorial for PD Particles, around this time last year (August 07), its located in the Software Discussion Forum.
If its free, you must get it! PD Particles can be used like a Paint program, with good tools for sky, sea and long landscapes. The app is great at create plantlife too! Seaweeds, bushes, top down trees, evergreens or leafy trees. Some of its brushes are kind of weird - Jeremiah Tree for example.
Best way to describe PD Particles, is that it paints using a particle physics where your brush establishes where "seed" is planted and an alogrithmic formula that carries your seed to followup effect and direction. Its difficult to describe, you have to try it and see what I mean.
I find altering color, shadow, highlights is not very intuitive, as well as how to change particle parameters. Other modifiers allow you to change direction - required to make plant objects become top downs instead horizontal views.
You can export PD files as transparent PNGs from the app itself.
If you use 3D apps to create landscape walk-throughs, PD Particles can create 2D sprites of trees and flowers that appear as 3D objects in your environment - a way to place plants in your scene that isn't too heavy with pologon count.
I paid $19.95 for it and thought it a worthwhile application - for free its something you need to try out now.
One last point, if you got kids, they will love how this proggie works, its fun to watch while you're painting.
PS: heres a few plant map objects I created using PD Particles... 2 trees and some kelp seaweed... (all transparent PNG files).