macosx-nosleep-extension – The MacOS X kernel extension, preventing sleep when you close the lid. – Google Project Hosting.
Very useful, especially when watching a movie on a projector using HDMI directly, instead of using AirPlay with an Apple TV. Thanks to NoSleep, you can close your MBA’s/MBP’s lid but still stream the movie to the projector.
If I were Apple, I’d be a bit embarrassed that such a setting is not already built-in.