FMA's MS2000 Functional Distress Marker is very useful in the airsoft world and could prove to be rather convenient while out in the field. The real steel version has IR light so ones with NVGs could see them whilst the opposition can't. This is the airsoft world and that's illegal.
This one has two lights and flashes every two seconds in the direction you're pointing in spurts which could be used as a tactical maneuver or just to show you're in distress. It uses 2 AA batteries which you insert from underneath and is very easy to use!