I got a few questions about this, and it seemed like something worth writing about... so here it is. My newest (usable) leaf brush:
I did most of the foliage in the image from the previous post using this. It's fairly simple in terms of dynamics with just a flip X and some slight size change. A little Angle and Color jitter might not be a bad thing here, but I let those be for now. I would attach the brush itself, but I believe very strongly in designing your own brushes, even if it means copying one you like as closely as possible. It's a good learning experience and helps in remembering what brushes are at your disposal without having to take the time and go look through a list (at least for me it).
Should someone choose to make a brush like this, I would note that there are no 'leaf' shapes here, only hard-edged blobs. This makes it look more realistic in your image, and more versatile for painting things that aren't leaves. Maybe a rock texture. For this type of thing I would also argue that it's important to keep the brush image dark, and to leave opacity at maximum to avoid any weird, and very digital looking transparent leaves.
Let me know if you guys have any more brush questions and I'll do my best to answer them. :)