SAOImage DS9
DS9 is an astronomical imaging and data visualization application. It supports FITS images and binary tables, multiple frame buffers, region manipulation, and many scale algorithms and colormaps. It provides for easy communication with external analysis tasks and is highly configurable and extensible via XPA and SAMP.
DS9 is available as source code, but it's quite complex to build because of the numerous dependencies. They provide a static binary as well, which makes it much more easy to use it.
It is developed by the Smithsonian Astrophysical Observatory (SAO).
DS9 is used by XmCCD and GoQat for captured image visualization.