BlockCAD

BlockCAD is a freeware program for building virtual models with Lego-like bricks.
You can save your models, or save pictures of them (.bmp, .jpg), even reuse a complete model as a part in another model.

BlockCAD is not a full CAD program, the pieces can only be placed 'studs up', and rotated in steps of 90 degrees. It is impossible to draw pieces that are partly inside other parts, which excludes almost all of the Lego Technic line, but there's still a lot of fun to be had with it.