From the webpage:
SAOImage DS9 is an astronomical imaging and data visualization application. DS9 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 a stand-alone application. It requires no installation or support files. All versions and platforms support a consistent set of GUI and functional capabilities.
DS9 supports advanced features such as 2-D, 3-D and RGB frame buffers, mosaic images, tiling, blinking, geometric markers, colormap manipulation, scaling, arbitrary zoom, cropping, rotation, pan, and a variety of coordinate systems.
The GUI for DS9 is user configurable. GUI elements such as the coordinate display, panner, magnifier, horizontal and vertical graphs, button bar, and color bar can be configured via menus or the command line.
New in Version 7
3-D Data Visualization
Previous versions of SAOImage DS9 would allow users to load 3-D data into traditional 2-D frames, and would allow users to step through successive z-dimension pixel slices of the data cube. To visualize 3-D data in DS9 v. 7.0, a new module, encompassed by the new Frame 3D option, allows users to load and view data cubes in multiple dimensions.
The new module implements a simple ray-trace algorithm. For each pixel on the screen, a ray is projected back into the view volume, based on the current viewing parameters, returning a data value if the ray intersects the FITS data cube. To determine the value returned, there are 2 methods available, Maximum Intensity Projection (MIP) and Average Intensity Projection (AIP). MIP returns the maximum value encountered, AIP returns an average of all values encountered. At this point, normal DS9 operations are applied, such as scaling, clipping and applying a color map.
Color Tags
The purpose of color tags are to highlight (or hide) certain values of data, regardless of the color map selected. The user creates, edits, and deletes color tags via the GUI. From the color parameters dialog, the user can load, save, and delete all color tags for that frame.
Cropping
DS9 now supports cropping the current image, via the GUI, command line, or XPA/SAMP in both 2-D and 3-D. The user may specify a rectangular region of the image data as a center and width/height in any coordinate system via the Crop Dialog, or can interactively select the region of the image to display by clicking and dragging while in Crop Mode.
I encountered SAOImage DS9 in the links section of an astroinformatics blog.
Good example of very high end image/data cube exploration/processing application.
You are likely to encounter a number of subjects worthy of comment using this application.