The [link=http://www2.towerhobbies.com/cgi-bin/wti0001p?&I=LXSZ87&P=ML]Air Alert[/link] is what I've used. Funny thing is... the one time I really needed it, my plane managed to shut itself off when it crashed, so the air alert wasn't working!

A better solution would probably be a beeper that is always on (or comes on after a fixed time period), and is physically attached to it's own battery or pack. In a crash, this setup will be unlikely to get powered down or shut off.