DS9 is free, open source software available for Windows, Mac or Linux. It can be used to scale and convert FITS files so they can be used in an image processor such as The Gimp or Photoshop. DS9 can also be used to analyze data from a telescope.

DS9 can be downloaded free from http://hea-www.harvard.edu/RD/ds9/

This pdf document will get you started using DS9 to view and analyze FITS files: DS9 Software