Here you will find my little collection of 3D plants, all created from scratch with ngPlant (I have also written a tutorial on plant-making with open-source software). Everything is under a creative commons license, which means you can download and reuse them any way you like, even in commercial works. Of course, I am always happy to know the great uses you do with them!
Note: Some of these plants are packed as blender files, and are available on the old blender greenhouse page.