I often hesitate to mention F/FOSS apps that have not seen active development for a couple of years. But, as a huge LEGO fan, I'll make an exception for this Windows Freeware app...

BlockCAD 3.18

...which was release over two years ago on January 4, 2005. It lets you build virtual models using LEGO-like bricks on your Windows screen.

Let me know if you have any favorite LEGO-related freeware or Open Source apps for Windows or Mac OS X.