There is just "ADOBE PHOTOSHOP CS", which I used, until it busted (I think it was my error)
It was great, simple as heck, and produced great results. Plus, it supports many formats that I can't get with freeware.
On the subject of freeware, try
http://www.gimp.net/ or
http://www.getpaint.net/. These are both quite good.
I use GIMP to open the PSD files from
www.master-skinnerz.com, remove the wireframe, and save it as a JPG. I then bring the wireframe back, and delete everything but it. I then save the wireframe as a PNG, so it keeps transparency, then I do the same for the black mask.
Now, I open up these files on Paint.NET, use the JPG as the base to paint upon, then add the wireframe and blackmask. Reselect the base layer, and use the tools to get skinning.
Or, just pay for Photoshop.