Open-source and currently in use by planetarium projectors, cross-platform 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. Be aware that it only runs 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 that 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 are a bit hard to find, but there is a nifty record feature to make your own planetarium shows. Some of the interface features could be revamped and improved, including the somewhat confusing options settings and the hard-to-see interface controls. 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, will be impressed with Stellarium. Fortunately, that could be any of us.