My Petzl E+ light has an SOS strobe, which was one of the many reasons I bought it. A strobe light would be cool, because it could also signal for you as you sleep.

Any other passive signaling ideas, for the tired or injured? Bright coloured clothing, SOS/HELP/HELF makers on the ground, flags flapping in the wind, colourful or relfective tarp for a shelter....