There's always the good-old tuning fork! Most of the ones I've had/seen were all A-440 (concert pitch), which is probably why people get in the habit of tuning to A. There's actually an ISO standard which specifies that A is 440Hz. I found that a bit interesting/funny really, having to deal with standards all the time at work. I may have to slip an ISO 16 reference in my next spec. at work to see if anyone catches it (LOL to self).