Celestia - Real-time 3D visualization of space
Stellarium - Stellarium is a free open source planetarium for your computer.
Satscape - A satellite tracking program for Windows, Mac OS and Linux.
Planetarium - A browser-based virtual planetarium of stars and planets, customizable to any location and time.
Heavens Above - Website for tracking Satellites, Space ships and other space objects.
SkyView - SkyView is powerful, easy-to-use app that answers questions about stars, constellations, and more.