DraftSight is a free 2D CAD fully compatible with Autocad.
The interface is extremely similar to Autocad so you can do all 2D operations on dwg cad files in a familiar ambient. Actually even some 3d operations like the modelling of surfaces are supported.
As a PortableApp it would finally permit you to show and edit and even create Autocadfiles wherever you are. An option that is really missing in PortableApps