Stellarium
FreeA planetarium on your desktop, showing exactly what your eyes would see
An open-source desktop planetarium. Give it a location and a time and it renders the sky exactly as it would appear, with hundreds of thousands of stars, planetary motion, constellation lines, eclipses and meteor showers. It can drive a telescope and can wind the sky thousands of years forwards or backwards. The Windows installer is about 456 MB.
Why we picked it
Stellarium is the kind of software you open once and then just watch for a while. It is not a diagram — everything is rendered from real astronomical data. Set it to eight tonight in your own city and the sky on screen is the sky you would see stepping outside, down to a simulation of how light pollution reduces visibility.
Its practical value is solving "there is a bright dot up there and I have no idea what it is." Check before heading out and you know which one is Jupiter, when the ISS passes over, and where a given constellation will be. Stargazers use it to plan a night's targets, and supported telescopes can be pointed directly from the app.
Time travel is the other delightful use: set the date to 3000 BC and see which star was the pole star then, or check whether an eclipse fell on the day of some historical event. Historians and writers occasionally need exactly this.
9,871 stars, GPL-2.0, fully offline on the desktop, no account required.
Do note the download size — the Windows installer runs around 456 MB and the Linux AppImage exceeds 540 MB. That weight is mostly star catalogues and high-resolution textures, so leave room and time. For a quick lookup there are lighter web and mobile versions with a reduced feature set.