Planets is a fun, interactive program for many-body gravity simulations. The emphasis is on play: it's easy to add in planets, zoom in and out, change the physical constants, save and load configurations, etc. It's designed to be easy enough for a kid to enjoy, but that hardly precludes adults.
Mars MOLA Viewer displays a fly-over rendering of the Martian surface, as measured by the Mars Global Surveyor, in real-time 3D animation. It downloads the Mars Orbiting Laser Altimeter topographic data files and uses them to drive the rendering. The user controls the position and the angle of the camera, and can move seamlessly over the entire Martian surface from -88 to 88 degrees latitude. It is a Java Web Start program with 1-click installation from your browser (depending on platform/browser).
Henry is an educational astrometry utility for performing calculations based on a Hertzsprung-Russel (H-R) diagram, such as spectroscopic parallax for working out the distance to a given star. Unlike many other solutions that use in-built images for their H-R diagrams, Henry plots data directly from the European Space Agency's HIPPARCOS catalog, allowing increased accuracy while also giving students greater room to experiment by exploring the catalogue themselves.
Open Source Astronomy is a comprehensive collection of astronomy software packages built for Linux, Mac OS X, and Windows. Each collection comprises approximately 5GB of software. An installation assistant is included along with a browsable documentation library covering all included packages.