Apart from their use in electronics, which we won't comment on,
	and for certain fluorescent lights (they won't turn on without
	a good ground connection), they're intended to guard against
	insulation failures within the device.  Generally, the case of
	the appliance is connected to the ground lead.  If there's an
	insulation failure that shorts the hot lead to the case, the
	ground lead conducts the electricity away safely (and possibly
	trips the circuit breaker in the process).  If the case is not
	grounded and such a short occurs, the case is live -- and if
	you touch it while you're grounded, you'll get zapped.  Of
	course, if the circuit is GFCI-protected, it will be a very
	tiny zap -- which is why you can use GFCIs to replace
	ungrounded outlets (both NEC and CEC).

	There are some appliances that should *never* be grounded.  In
	particular, that applies to toasters and anything else with
	exposed conductors.  Consider:  if you touch the heating
	electrode in a toaster, and you're not grounded, nothing will
	happen.  If you're slightly grounded, you'll get a small shock;
	the resistance will be too high.  But if the case were
	grounded, and you were holding it, you'd be the perfect path to
	ground...

Some parts © 2009 Advameg, Inc.