I'd like a device that let's people hear what's coming out of their mouths. Honestly, sometimes people say stuff that doesn't make any sense to anyone but them. I'd like to have some sort of translator that can playback what they said, as it's heard by other people so they can have the confused-dog-head-cocked-to-the-side look that everybody else is giving them.
In h.s., I had to invent a product to sell, so I made up this thing called spitfire, a chemical that you can drop in your drinks at restaurants. It eats impurities, so if you call out a rude waitress and she spits in your drink, you don't even have to worry about it.