World Wide Telescope
From web to desktop to full dome planetarium, WorldWide Telescope (WWT) enables you to explore the universe, bringing together imagery from the best ground and space-based telescopes in the world and combining it with 3D navigation.This software is free!
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.
MayaCal is a free calendar application for exploring the Maya, Gregorian and Julian calendar systems. Only available for Macs.