I agree that Gimp is one of the best free image manipulation programs out there. It is also available on multipule operating systems (Linux, Solaris, Mac OSX and Windows)
ImageJ is another option which is less "photoshop like" than Gimp, but has some interesting options for manipulating scientific images. ImageJ is a public domain Java based image manipulation program developed by the U.S. National Institute for Health (NIH.)
rsb.info.nih.gov/ij/
  