Gaia Sky is a real-time, 3D, astronomy visualisation software
Siril
A free astronomical image processing software
Marble
Marble is a virtual globe and world atlas — your swiss army knife for maps that you can use to learn more about the Earth and other planets.
Stellarium
Stellarium is a free open source planetarium for your computer.
KStars
KStars is a Desktop Planetarium by KDE. It provides an accurate graphical simulation of the night sky, from any location on Earth, at any date and time.
Celestia
Celestia – real-time 3D visualization of space