Celestia - Real-time 3D visualization of space
Satscape - A satellite tracking program for Windows, Mac OS and Linux.
Heavens Above - Website for tracking Satellites, Space ships and other space objects.
See A Satellite Tonight - Free web app to explore satellite locations.
ISS Detector - Tracking the International Space Station. See it fly by.
Stellarium Portable - Stellarium Portable is software that you can use to observe the universe in a virtual 3D space.