LeoCAD is a CAD program that can be used to create virtual LEGO models.

LeoCAD has an intuitive interface that is designed to allow new users to start
creating new models without having to spend too much time learning the
application.

LeoCAD is fully compatible with the LDraw Standard and related tools, and it
reads and writes LDR and MPD files so that you can share and download models
from the Internet.

It currently features several thousand different pieces created by the LDraw
community.