Or rather: how can I disable the built-in display and carry on working on an external monitor without having the laptop overheat and whine like an aeroplane because the lid is down?
OS X users have spilt a lot of pixels on this and I too was frustrated because my shiny new 34" screen made the laptop screen redundant; but keeping the lid shut often made the fan turn on.
SwitchResX came to my rescue, not for the first time. It has a daemon option with a menubar icon for turning displays on/off & for switching the resolution.
I got SwitchResX originally because my monitor does picture-by-picture display of 2 computers plugged in to it at once; which is only truly cool if said computers have a screen resolution option for half-a-screen. Which SwitchResX solves by allowing custom resolutions. So I've become a fan: it works simply and reliably and solves my problems.
I only recently came across this dance http://osxdaily.com/2012/06/15/yet-another-way-to-turn-off-internal-lcd-display-of-macbook-pro-with-lid-open/ which I haven't yet tried. I know other attempts to 'trick' the screen off failed after Mavericks