QFitsView is a free application that allows you to visualize FITS files.
QFitsView is similar to SAOimage, DS9, and so on and was written using the cross-platform QT library and thus runs under Microsoft Windows as well as many flavors of UNIX and MAC OS-X.
In addition, QFitsView provides the complete DPUSER - The Next Generation application.
QFitsView is cross-platform and it works on Mac OS X, Windows and Linux. Binaries for the Windows and Linux platforms are available on the project's homepage.