Astronomy for fun (education, science, whatever). Nightfall is an interactive application to simulate eclipsing binary stars, and to produce animated views, synthetic lightcurves and more. Takes into account the non-spherical shape of close binary stars, mutual reflection, and some other effects. Comes with documentation, on-line help, and lots of observational data of real binary stars. Supports the Gnome desktop, but doesn't require it.
Gpredict is a real time satellite tracking and orbit prediction program. Besides the general orbital data for satellites, gpredict can also calculate the footprint, visibility, doppler shift, signal loss, and signal delay for each satellite relative to one or more ground stations. The calculated data can be viewed in tables, on maps, or on polar graphs.
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.
PP3 creates celestial charts. It generates resolution-independent maps of very high graphical quality. They can be used, for example, as illustrations in books or on Web pages. It is possible to change many parameters, and arbitrary text can be placed on the maps. The output formats are EPS or PDF.
NOVAS (Naval Observatory Vector Astrometry Subroutines) is an integrated package of subroutines for the computation of a wide variety of common astrometric quantities and transformations. It can provide the instantaneous coordinates (apparent, topocentric, or astrometric place) of any star or planet, and also provides general astrometric utility transformations, such as those for precession, nutation, aberration, parallax, etc. It is useful for data reduction programs, telescope control systems, and simulations.
ESO-MIDAS (European Southern Observatory Munich Image Data Analysis System) provides general tools for image processing and data reduction with an emphasis on astronomical applications, including imaging and special reduction packages for ESO instrumentation at La Silla and the VLT at Paranal. It also contains an applications packages for stellar and surface photometry, image sharpening and decomposition, statistics, and more.