XFrog Inc. has made available for you to download 130 free 3d plant and tree models from each of the 30 XfrogPlants libraries! Three to nine 3d plant models are included in each of the 30 species they provide (with all major formats included), and best of all, they are completely FREE! Download the plants, browse their documentation PDFs and try in your favorite software.
An important tip, regarding the rendering of opacity mapped leaves (In VRay, but probably in most other render engines too), is to make sure you remove filtering in the alpha/opacity map. This is crucial to make alpha mapped leaves render faster. If you are using Forest Pack Pro, then you have a built in Material Optimizer that can take of this for you for faster renders.
Now that you know that… Go ahead and explore them out!