If the main link doesn't appear to be working. It's:http://ldd.lego.com/
My sons love this and even better its free!
The Lego Digital Designer program that lets you build with LEGO bricks on your computer.
LEGO Digital Designer is a 3D modeling tool that enables you to design and render your own Lego creations. You can get started with a collection of sample models that can be customized and extended, or you can start your creation from scratch using a complete assortment of Lego bricks and your imagination. Simply drag and drop bricks into place and use the provide tool options to build your model.