Celestia - Real-time 3D visualization of space
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.
Orbitron - Orbitron - Satellite Tracking System written by Sebastian Stoff.
Stellarium Portable - Stellarium Portable is software that you can use to observe the universe in a virtual 3D space.