Xplns is a free astronomy simulation software written specifically for Unix. It reproduces real starry sky on your display of X Window System. It calculates the position of many celestial objects accurately. You can enjoy the past and future sky on the desk top as if you have traveled all around the world.
StarPlot is a GTK-based program, written in C++, which can be used interactively to view three-dimensional perspective charts of stars. Charts can be recentered, rotated, or zoomed in or out with a mouse click (this can also, of course, be done via dialog boxes for more precision). Stars may be viewed (or ignored) by spectral class and absolute magnitude. Other features include support for both celestial and galactic coordinate systems, the ability to display extended non-stellar objects, and a pop-up Hertzsprung-Russell diagram. StarPlot is also packaged with starconvert, a utility which converts line-oriented stellar data records to StarPlot format. Most star data files available on the Internet can be converted this way if a short file describing the original file format is provided to starconvert.
The FLASH code is a modular, adaptive, parallel simulation code capable of handling general compressible flow problems in astrophysical environments. It has been designed to allow users to configure initial and boundary conditions, change algorithms, and add new physical effects with minimal effort. It uses the PARAMESH library to manage a block-structured adaptive grid, placing resolution elements only where they are needed most. It uses the Message-Passing Interface (MPI) library to achieve portability and scalability on a variety of different message-passing parallel computers.
Xgravity is a program for the simulation of 3D gravitational motion of n-bodies. It reads a file with the current state of the bodies, and displays the motion. It features a nice isometric perspective, Red-Green stereographics, and a stereographic perspective which allows you to see the trace in "real" 3D. There is a GTK GUI interface in development, which gives you the ability to perform basic operations with the system, like modifing objects, rotating, eliminating drift, and launching a simulation.