When practicing astrophotography, knowing what is up in the sky is of course essential. You need to know where an object is in order to photograph it, and even if you just take wide field photos of the stars, it is nice to be able to identify the brightest of them. I use the program Stellarium for this.
Stellarium is a free open source planetarium that renders the skies in real time, and it works under both Windows, OSX and Linux. Using it is very easy. You select your location and the time, and then you are good to go. You look around by simple mousedragging. The constellations, stars, planets and other objects are all labeled for easy identification. You can even zoom in to see objects such as the planets and the nebulae up close.
Stellarium contains all the stars from the Hipparcos catalogue (more than 100,000), the planets of our solar system and their natural satellites and all Messier and NGC objects. From the configuration menu it is possible to download star catalog updates and thereby populate the sky with millions of stars. You can also activate a plugin, that displays the orbits of artificial satellites such as the international space station.
All in all Stellarium is a very nice piece of software. It lacks one thing, though. I does not inform you about special events such as meteor showers, comet passages and planet oppositions, all of which make up very interesting motives. For that you have to keep an eye on web sites such as Sea and Sky.