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.
Celestia - Real-time 3D visualization of space
SkyView - SkyView is powerful, easy-to-use app that answers questions about stars, constellations, and more.
ISS Detector - Tracking the International Space Station. See it fly by.