SaVi 1.4.1 is now available. SaVi lets you simulate satellite orbits and coverage, and includes simulations of complete systems such as GPS, Galileo, Iridium, or 03b Networks. SaVi needs only Tcl/Tk running on unix. 3D rendering of satellites requires Geomview, another SourceForge project.
The SaVi 1.4.1 release fixes editing problems with 1.4.0, and adds a complete help and documentation system that describes and explains the available simulated constellations. It's a lot more polished than 1.4.0.