Blender is an open source multi platform 3d modeling program. http://www.blender.org

Blender Interface

Navigating in 3D

Mouse roller zooms in.

Lower left button (cross here): refers to which mode you are in:

Press middle mouse button to scroll menus/headers.

To move an object:

Left click, move and release. This will start to move objects.

More operations, like select, duplicate:

Left mouse gets the pointer to where new objects will be inserted.

Click n. You can snap things to grid.

Say you have parts on a screen. How do you align them?