Software specific to astronomy. Some of this I've tried, some I haven't but would one day like to.
DeepSkyStacker is a freeware for astrophotographers that simplifies all the pre-processing steps of deep sky pictures
Affordable, Powerful, and Easy to Use Astrophotography Software
Free software for alignment/stacking/processing of images.
The ASCOM Initiative is a loosely-knit group of developers and astronomical instrument makers that work together to bring vendor-independent and language-independent plug-and play compatibility between astronomy software and astronomical instruments on Windows computers. ASCOM stands for the Astronomy Common Object Model.
Stellarium is a free open source planetarium for your computer. It shows a realistic sky in 3D, just like what you see with the naked eye, binoculars or a telescope.