Screensaver

Electric Sheep

Immersive, trippy screensaver.
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Planetariums and Outer Space

Celestia

Open-source 3D planetarium software. Akin to traveling through space (outer space).
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Stellarium

"Renders realistic skys in realtime." See what you would see if your city wasn't so light-polluted. Used in real planetariums.
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Also see: macORSA - Orbit Reconstruction, Simulation, and Analysis

Comic Viewer

Jomic

Read digital comics.
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Dance Dance Revolution

Stepmania

Dance Dance Revolution clone. Can connect to external dance pads.
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Games

Cube

Nice first-person shooter.
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Marathon: Aleph One

Engine for Bungie's free FPS series Marathon. Play online, solo play with rich original story, many total conversions and thousands of maps.
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Zombies

3D zombie strategy game.
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Armagetron Advanced

3D Tron game.
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BZFlag

First-person tank game.
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Battle for Wesnoth

Fantasy / Strategy Game
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Also see: Snes9X (SNES emulator), Frozen Bubble, PySol for Mac (card games), PLANESHIFT (3D Fantasy MMORPG), VDrift (drift racing simulation).