Underwater Strobe/Flashlight - bought one of these for my BC shoulder strap. Great task light if you're doing a lot of night dives and you need both hands free. Only used the strobe once (safety reasons) on the surface - but that's a story for another time! ;-)
Ambulance Rip Shears - Depending on where you dive, it helps to have a pair of these in your BC pocket. Fishing line underwater is invisible and hard to cut even with a conventional dive knife. A must have.