Free 3d Garden Design Software
In my own work, I've worked with Google Sketchup because it's a free program that is extremely powerful and once learned, can be used for all kinds of landscaping or design work. A full modeling suite of software tools, this software is easily adapted to landscape design drawing.
Please understand there is a learning curve with this; you can have a ton of fun but it will likely take you a few hours to master the basics. Luckily there are learning tutorials on the site and following along with those and doing the exercises is well worth the time it takes.
There is also a warehouse of models that are already constructed so you don't have to spend time making benches, swings, trees etc to fill your work.
Finally, I did purchase a book from Amazon, Google SketchUp for Site Design
This book walks you through the basic uses of the software and step-by-step helps you create all the shapes and layouts for a landscape. It took me some practice time but between the tutorials and the book, you get a complete 3d garden design package for a few dollars.
If you're really strapped for cash, just go with the free download and the tutorials.
This video is of a virtual garden that was constructed using Google Sketchup. (I didn't do this one but did have it running on one of my servers for a time to play around with)
