LDraw is a suite of tools designed to allow anyone to manipulate virtual bricks on their computer. It was originally designed by the late James Jessiman, and has been continued after his untimely death by enthusiasts around the world.
You can check out all about it at LDraw.org. My contributions have been fairly limited, the parts I have authored can be seen here (note that these are my versions, they may have been subsequently improved by others).
Here is a list of all my LDraw related projects: