Source model skin editor
Skin Editor is a program written by hlstriker that will let you add, edit, or delete skins on a Source engine model file. This tool is useful if you want to modify skins on a model but don't want to take the time to decompile, possibly rebuild the .qc file, and recompile the .mdl file.
Tutorial usage videos
You can download the latest working version here: http://swoobles.com/forums/thread-4545.html