LDraw is a set of free tools for creating 3D virtual lego models.
You're able to build new models, record models you've physically built, view the creations of others, render photorealistic images of your models, even make animations.
The program comes with 175 pre-built models to give you an immediate idea what's possible. Launch the viewer (LDView) and browse folders including "Lord of the Rings", "Pirates", "Simpsons", "Star Wars" and more.
Opening the same file into LDCad enables viewing and editing of the model. This is a powerful tool with CAD-like features and functionality, so you should expect a steep learning curve, but just left and right-clicking objects and browsing the menus will let you see what's possible.
There are plenty of other tools included - editors, viewers, converters, parts libraries and more - but again, don't expect to figure them out in a hurry. LDraw has been developed over 20 years by multiple people, and there's now a lot to explore.
LDraw is a hugely powerful set of Lego modelling tools. You'll need some experience with CAD or modelling programs to get the most from it, but Lego fans can also just spend half an hour browsing the sample models.