Stellarium
I'm a sucker for good GPL applications, and this one really is one of a kind, Like many other programs out there (that costs a ton of hard earned cash) it is a live map of the sky. Some of the more interesting features is that you set your location in Longitude and Latitude (can be found either in Google Earth or with a handy GPS) then it will display how the sky looks at your location.
It has a ton of fun features with a good sized database of objects, you can zoom in and out (the graphics aren't as spectacular as the Sky Mode in Google Earth but good enough) lock onto a target (star, nebulae, planet, etc.) and have the system track it.
You can go back and forth in time (the algorithms are estimated to be accurate for at least a thousand years into the future) and see how the sky looked yesterday, or this time next year.
It has automatic constellation view, that lets you toggle the lines connecting the stars of the constellations, you can toggle the constellations themselves and if you feel like it toggle artistic interpretations of the constellations.
It also has several very exotic features, such as a special projector feature that allows you to create your own planetarium with a projector and a spherical mirror.
The ability to control up to 10 motorized Telescope mounts, (such as NextStar and LX200 mounts) yes control, meaning you select a star and the send the command through the program to a telescope server that in turn translates it into commands for the telescope to point at that star (forget the measly databases that comes with these kinds of mounts, Stellarium has over 600,000 objects in it's default intall database, this database can in turn, easily be extended A LOT further..)
Among all these features is a very easy and intuitive GUI that allows you to cruise the sky's and find your way among the stars.
