Google Sketchup is a 3D drawing program available free from Google. Its a great tool for modeling timber frame designs in 3D, and you can't beat that price! You can create simple drawings where timbers are represented as simple rectangular solids. This is often all you need to demonstrate to your clients what their frame will look like. But Sketchup is capable of doing much more. You can actually make a highly detailed model that includes all of your joinery design. You can then create accurate shop drawings for laying out timbers in your shop.