I have these big red emergency off switches on all my printers.
http://m.ebay.co.uk/itm/Emergency-stop-button-station-twist-to-release-CE-1-N-C-contact-PT-A1-/231146690965?nav=SEARCH
They are only single pole so for mains heat beds in an error state caution is still advised.
I also with a bit of help from David and a few other on reprap came up with a simple smoke power shutoff.
http://forums.reprap.org/read.php?392,578686
It's not been extensively tested to determine if it will shut off the power before a fire begins, but it does latch in an off state when smoke is detected and requires manually resetting once the fault has been cleared, I expect it would work best in an enclosure. No false triggers yet after over a year in use, no genuine one either thankfully. It's obviously independent of the controller, and fairly simple.
What I have started using on my new printer is a plug-in rcd (ground fault device) right next to the machine. I can hit the test button as an emergency stop or general off switch, it should protect me against a mains heat bed problem and its double pole so shuts off the power to the device completely.