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.
Heavens Above - Website for tracking Satellites, Space ships and other space objects.
Stellarium - Stellarium is a free open source planetarium for your computer.
See A Satellite Tonight - Free web app to explore satellite locations.