Sound pressure level meter. They measure the sound pressure level (i.e. the volume of something in decibels). You get them at Radio Shack. It's $35 for an oldskool analog one or $50 for a digital one. The analog ones are better (faster response and generally more accurate).

They're great for calibrating amplifiers and speakers, and telling people (such as noisy roommates) that they need to quiet a bit. ("Listen, I'm trying to sleep, but you're generating 90 decibels of noise in my room. I have an SPL meter to prove it.")

Very useful. I like to calibrate my stereo system every few weeks, and I also take it to random places where someone might want their own stereo calibrated (it helps to have a pink noise generator). They are usually quite grateful since before calibration they just have to do it by ear, but after calibration they know they're getting an accurate soundfield (energy-wise, anyway).