As a kid my dream was to have so many bricks that I never run out of them during building. As time has passed I have realized that this is never going to happen. But there is a small hope - you always have enough when building from virtual bricks.
This post promotes LDraw. This is a free program that can be used just like some serious piece of 3D CAD software - to build models from Lego bricks. For example like that:
Matt Wagner, a famous Lego builder had a guest editorial on the Brothers Brick blog titled Five things you may not know about LDraw . It is worth reading, I think it is a great intro to LDraw.
Besides LDView you can also learn how to make raytraced images using Koyan's tutorial on how to render LDraw images with POV-Ray.
Now, if you happen to browse Brickshelf and find some .LDR files - you should know what it is!
(As a sidenote, Lego has their own official building program: Lego Digital Designer. I'll write about it another time.)