Open source and currently in use by planetarium projectors, Stellarium brings astronomer-level features to stargazers of all levels of interest. It's not quite as robust as its competitors, but it's also a much faster program. It doesn't suck away your RAM into a black hole when loading or running. It does run only in full screen mode, making any other programs you're running inaccessible except for the ALT-Tab switcher.
The default catalog includes 600,000 stars, with upgrade modules that can push the count up to 210 million stars. The constellations of 10 different cultures are included, as well as illustrations and asterisms to help you visualize what the ancients saw. There's a full Messier catalog of nebulae, too. The dawn, dusk and atmosphere backgrounds were good, but not great on our monitor. They probably look better on a planetarium dome, which is why it's useful that Stellarium also includes a fish-eye view for curved surfaces. Besides equatorial and azimuthal grids, users also get shooting stars when appropriate, eclipse simulation, and skinnable landscapes. Stellarium incorporates star-views from the Moon.
The controls live in the lower left corner and are transparent--a bit hard to find. The nifty record feature is somewhat hampered by the dark interface. When you run the program for the first time it asks that you set your current location, but the mouse-over map of the world was too small to use easily. Stellarium should appeal both to users who need something more academic and less distracting than Google or Microsoft's offerings, as well as those who have a need for an open-source planetarium. Fortunately, that could be any of us.