Take a look in here:
http://www.eecs.wsu.edu/paint.net/
A screenshot:
It's not as versatile as Photoshop, Paintshop or Gimp, though, but I found this software as a great and simple learning tool for skinning. There's no "remove white"-ish plugins yet, but they also give the source code for anyone to make their own filters, so I guess it's only a matter of time until someone write a remove white - kind of - filter and other skinning related filters for this program.
