In Brick x Brick, we take advantage of free software Sketchup by Google. It's a 3D drafting program anyone with a computer can download from the internet and use. Tutorials come with it to show you how.
Another one we love is Lego Digital Designer. It's free, and lets us get creative with Lego building blocks like we used to do on the living room floor - but in a digital environment. Download it here.