Multi-body Gravity Simulator (MGS) allows you to perform a simulation based on a fixed time slice. Its unique features are the ability to perform realtime simulations, a three dimensional visualization of the simulation, and the ability to design simulations in XML and with a GUI. The accuracy of realtime simulations will depend on your machine speed. Fixed time step simulations are also supported.
MultiSeti is a utility for the Seti@Home client which helps you manage multiple SETI packets. If you have dial-up access to the Net, want to download more than one packet, and don't want to run clients simultaneously, MultiSeti will run one after another (when one is finished analyzing, it starts seti to analyze the next packet).
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.
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.
Nova is a free Integrated Observational Environment for astronomers. It combines Ephemeris functionality with the capability of controlling modern "goto" telescopes and popular CCD cameras. Nova combines image capture, telescope control and ephemeris functionality into a single application. The main goal of Nova is to make it easy(ish) to capture CCD images of objects with a good degree of accuracy. Nova is also able to manage several kinds of astronomical database (i.e GSC, USNO, SAO, etc.) and makes it easy to automatically schedule observational activity through the use of target object lists.