Execution by having stones thrown at one by others. Biblical punishment still applied in the following countries: United Arab Emirates, Saudi Arabia, Iran, Afghanistan, Pakistan, and Sudan.
The condemned, at least in modern times, is wrapped in a white shroud, and buried upright up to the neck (women; to hide the breasts) or armpits (men). The crowd of onlookers then pelts the condemned with stones, larger than pebbles, but not large enough to that two would kill instantly. In an interesting twist, according to Islamic tradition, if the condemned can pull themselves free during the stoning, they are acquitted. (This is reportedly very uncommon, though.)
Do a google search on 'stoned to death', and you'll most likely find a lovely video of four people being stoned. Gruesome's a pretty accurate description.