Windows/Mac: Freeware application LEGO Digital Designer is a virtual LEGO kit for your Windows or Mac desktop. Once installed, you can either use LDD to build your own masterpiece from scratch or—if you're lacking patience—you can get a head start by using one of their starter models.
With over 763 brick types to choose from, your LDD-design will have reached well beyond the limits of your normal LEGO kit. Once you've built the perfect prototype, you can upload the results to the LEGO web site to order a custom kit with every brick you'll need included! LEGO Digital Designer is freeware, Windows and Mac only.