Stellarium. It's planetarium software that runs full screen and literally shows you where everything is in the sky, you set a place, time, and optionally a level of light pollution, and it renders what the sky would look like under those conditions.
- GPLv2 licensing, so it meets your cost requirements.
- Support for Windows, macOS, Linux, and BSD with minimal differences in any of the interfaces.
- Support for viewing from any location on earth (though it needs a bit of extra help to account for the local skyline), as well as from any position on a couple of other celestial bodies.
- Reasonably accurate support for determining the approximate position of stellar objects in the sky in the distant future and distant past.
- Easy output of coordinates in the sky for objects in multiple coordinate systems, with integration for some powered telescopes.
- A built-in red-light filter mode for usage outside at night without significantly impacting the viewing conditions.
- A collection of constellation databases covering more than a dozen stellar mythologies, with some rather nice art of the constellations themselves.
- The ability to turn off the 'ground' so you can see the relative location of objects below the horizon.
- The ability to turn off the 'sun' so that you can see the position of objects in the sky during the day.
- Support for tracking satellites, the ISS, and (I think) some other man-made objects.
- Support for operating with an arbitrary skyline. You can take a panoramic photo of the area you will be viewing the sky from, and with a little work then import that into Stellarium so that you can see what's really visible.
They also do a good job of separating the stellar object databases from the main application, so it takes up very little space with the default install, and you can easily download progressively larger databases to add more and more to what it displays (though going past about the first 3 is generally overkill if you're just doing naked-eye observations, and past about 7 is largely pointless if you're not operating a really big telescope).