Openuniverse supercedes the older package Ssystem. The following comment about Ssystem usability may or may not still be true of Openuniverse.
A solar system and celestial body viewer, Ssystem uses ?OpenGL models of the planets and various other objects. A 3d video card is recommended, but it is quite useable (at ~3fps) on my P133 machine. The interface is a bit difficult and confusing.