Robot App™ Description
LeoCAD is a CAD program that can be used to create virtual LEGO models. It has an easy to use interface and currently features over 4000 different pieces.
Leonardo Zide has tirelessly worked on his LeoCAD, a Windows and Linux editor for LDraw models. He's incorporated his own really cool features too.
For those of you craving mouse support, this is the program for you. After some getting used to, you can use the mouse to place parts in all 3 axes, or you can select which axis you would like to lock right on the toolbar to make it easier. Click and hold the left (primary) mouse button on a part and move it in the Y-Z plane, hold the secondary (right) button on a part and move it vertically along the X-axis.