For those of you interested in designing Lego models who want the freedom of not tying up all your bricks while you design, I highly recommend Lego Digital Designer. It’s a great, free program from Lego that contains its entire virtual catalog of parts. It lets you save images and generate building instructions. With additional, also freely available extensions, you can also generate parts lists. You can export LDD files to other Lego building software programs including LDRAW. The only downside is that Lego discontinued CREATOR, which allowed you to upload your designs to Lego.com and to purchase custom sets based on those designs.
Using LDD, I am prototyping many of by builds before I construct them for a variety of reasons. I like the CAD feel of the design process, and I have already discovered my designs are improving by developing them electronically first. Below is screenshot of a nano model of the bank barn on my farm rendered in LDD: DLH
[Update: now with links to LDD and LDRAW!]