The free viewer of AutoCAD drawings - Autodesk DWG TrueView (see www.cadstudio.cz/download) - can be tricked to be used as an editor of DWG files. This trick is based on the hidden commands method described in the tip 6342.
In the command line of DWG TrueView 2010 or 2011 (this trick does not work in the older and newer versions), insert - using copy/paste - the commands GRIPS and -WBLOCK. With these two non-blocked commands you can edit your drawing and save it.
First use GRIPS to enable the grip editing. Now you can use grips to edit, move, rotate your drawing objects. The standard save commands are disabled but the command -WBLOCK still works. By pasting this command you invoke the block export function, where you can choose the DWG or DXF file format, and specify the export of the whole drawing.
It is an undocumented and unsupported function of the DWG viewer.