Lego Digital Designer with NXT parts...
Check out the news here. Download and try it out here.
From the news: "The latest release of LDD has all the LEGO elements found in the MINDSTORMS NXT set #8527 so you can build robotic inventions in 3-D. Also included are three sample MINDSTORMS models to help get you started. They are exclusive attachments you can add to Tribot, and they can only be found on this version of LDD."
Please be aware that the installer for the Mac version is having some issues and is being worked on...
UPDATE: Here is a link to download a 5MB Word document containing a simple walkthrough/tutorial for using LDD and some of the basic controls. I may add more details to it, so please let me know what you think should be added. Thanks. - Jim