LEGO Digital Designer is a program that allows you to design and build LEGO models from a somewhat limited parts library. It's similar to the non-affiliated LCAD system, but a little more polished. The best part is that it integrated with LEGO Factory, so you can buy the models you have designed digitally and have them shipped to you. Still, parts go out of stock and LEGO is never cheap, so you can build things that are impractical to buy.