2 additional freeware programs, one which might interest more people, and the other is pretty specific:
Stellarium
Stellarium
A planetarium freeware program. Plug in your coordinates, it gets hour/date from your computer, and voila, you can see the stars, nebulas and planets outside, complete with names, constellations, coordinates, distance and other interesting info for the amateur astronomer.
Deep sky stacker:
DeepSkyStacker - Free
For those interested in astro photography, this program automatically aligns multiple pictures if you are doing the multiple exposure/stacking thing