Cactus is a general, modular, parallel environment for solving systems of partial differential equations. The code has been developed over many years by a large international collaboration of numerical relativity and computational science research groups and can be used to provide a portable platform for solving any system of partial differential equations.
libAstronomy provides a set of astronomical routines to ease the calculation of ephemerides. It includes mathematical functions to compensate various astronomical phenomena like precession, nutation, etc. The getconstellation routine finds the constellation for a given coordinate. It also calculates the positions of the sun and moon in two different precise ways.
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).
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.