Personally, I suggest SweetHome 3D - it's open-source (free) and very capable - can even export video tours of the finished product!
Why does the U.S.A. allow some terrorists to walk free in Miami, while imprisoning anti-terrorist fighters? (Washington Post) The Cuban Revolution as Socialist Human DevelopmentBrill Books (Amazon Paperback)
In a pinch, there's sketchup (free for the basic version). More a general 3D design application, but it can be used for deck design. (I designed an arbour for a deck with sketchup. Granted, I never built it, but I designed it.)