FITS Image Viewers
[free/open source - Windows, Macintosh, Unix] Download FITS viewers for various platforms.
http://www.cv.nrao.edu/~bcotton/fitsview.html

A Brief Introduction to FITS
[free/open source - Windows, Macintosh, Unix] Download FITS viewers for various platforms.
http://fits.gsfc.nasa.gov/fits_intro.html

FITS archive at NRAO
This is an archive for the Flexible Image Transport System [FITS], the standard data interchange and archival format of the worldwide astronomy community.
http://www.cv.nrao.edu/fits/

Flexible Image Transport System (FITS) Support Office
The format adopted by the astronomical community for data interchange and archival storage. Responsible for documenting the FITS standard, participating in its evolution, and advising NASA astrophysics missions on how to present their data in this type of format.
http://fits.gsfc.nasa.gov/