Stellarium - Stellarium is a free open source planetarium for your computer.
Space Engine - Space Engine is a realistic virtual Universe you can explore on your computer.
Celestia - Real-time 3D visualization of space
OpenSpace - OpenSpace is open source interactive data visualization software designed to visualize the entire known universe and portray our ongoing efforts to investigate the cosmos.
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.
KStars - KStars is a Desktop Planetarium for KDE.