LEGO Digital Designer
Also known as LDD.
LEGO Digital Designer is LEGO's own foray into the virtual building space. Compared to its primary competitor - the community created LDraw - it's often considered slightly easier to use due to the auto-snapping feature. LDD began life as a part of a program that enabled users to upload a creation and get a custom box and instructions to go along - essentially a way to create a custom LEGO kit. The program has since been discontinued, but LDD remains.
Swooshable has looked under the hood of LDD to see what parts and part variations are available. We share this data in our parts section.