Just to clarify a couple things from above:
Free just means no cost, open source means you can get and modify the source code. Just because something's free, doesn't mean it's open source.
Sketchup was originally released in 2000 by @Last Software, which was aquired by Google in 2007, and then a company called Trimble bought it from Google about a year ago. There's a free version, and a pro version that costs money.
You can find out more about Sketchup it here
http://www.sketchup.com, and there's some info about using it for landscape design here
http://www.sketchup.com/3Dfor/landscape-architecture