Worldsdesigner
They are completely free to use, just make sure to tell your friends about them
Each pack contains 100 different trees that you can import and use in worldsdesigner or worldpainter.
For tutorials on how to import them into your program of choice look at our guides for worldsdesigner or for worldpainter.
These treepacks are work in progress which means there will be more to come than there are listed at the moment.