Nice feature in Gimp. "Color to Alpha", huh? I'll have to look at that package more closely.

Of course that would only work with a solid background, but if that's all that you needed, it's certainly simple. The beauty of Mask Pro and Knock Out is that they can do it even with complex photographic backgrounds. Lemme see if I can show you an example...

Okay, this is just a real quick one in Knock Out, done by taking all the defaults:



There are tweaks you can do to make it even better, for example, preserving the hair detail even more. I just did that one really fast.