I had an idea similar to the ore sensors, but instead it's a "sounder" that finds air spaces! You right click it against a solid block face and it tells you how many blocks away in that (orthogonal) direction is the next air block (or null if there isn't one in range). There'd be two modes (m-right click to switch); one has a range of 10 blocks but uses a small amount of energy, and the other has a range of 64 blocks and uses much more energy.
It could be used to detect caves (like if you built a house and you keep hearing weird noises from underground, you can find where they're coming from), connect tunnels and the like. I personally like making my house on a firm foundation so I'd check to make sure there's no holes underneath.
Crafting recipe? I have no idea, you balance it.