Celestia - Real-time 3D visualization of space
Space Engine - Space Engine is a realistic virtual Universe you can explore on your computer.
Satscape - A satellite tracking program for Windows, Mac OS and Linux.
OpenSpace - OpenSpace is open source interactive data visualization software designed to visualize the entire known universe and portray our ongoing efforts to investigate the cosmos.
Heavens Above - Website for tracking Satellites, Space ships and other space objects.
See A Satellite Tonight - Free web app to explore satellite locations.