Star Chart - Stellarium is a planetarium software that shows exactly what you see when you look up at the stars. It's easy to use, and free. Read more about Star Chart.
Space Engine - Space Engine is a realistic virtual Universe you can explore on your computer.
Google Sky Map - Google Sky Map is a free hand-held planetarium that allows you to identify stars, planets and more.
Celestia - Real-time 3D visualization of space
Code NASA - 253 NASA open source software projects
Stellarium - Stellarium is a free open source planetarium for your computer.