The Scubapro Air Blow Gun is one of those little accessories that, until you've used one, you have no idea what you've been missing out on! This nifty bit of kit simply connects to the BCD hose on your regs and allows you to blow compressed air in a controlled and directional way.
Perfect for drying gear, cleaning sand from equipment, cleaning threads / grooves (like your cylinder valves)! You can even use it to refill the pontoons on your inflatable if someone has gotten a bit 'enthusiastic' with the bung!
Also a firm favourite of Technical Diving Instructors to make their students pretend that their gear is leaking, so that they can demonstrate how they would deal with it underwater.