1) Drinkable Mouthwash - We're all thirsty in the morning and we all have bad breath (some worse than others). This solution would solve both problems with a single gulp.
2) True 3D TV - Not one that you have to wear glasses with, or sit a certain distance away from the TV. I know there's a 20" 3D TV being made for Japan that will not require glasses, but you also have to sit a specific distance from the TV and within a specific viewing area. A better invention is obviously still needed (one that wouldn't require glasses and would be available in large-screen varieties).
3) Sober Up - This would be a liquid that would be consumed to instantly (within a few minutes) become sober. It would actually lower your blood-alcohol level which would of course make driving much safer. Of course, this would probably cause some people to drink way too much and increase the amount of liver failures in the world, but it's a start.
4) Portable Lie Detector - A lie detector that would be 100% accurate and would be roughly the size of an MP3 player (you also wouldn't need to attach it to your body). Think of all the ways this could be used, from negotiating with a used car salesman to political debates, the possibilities would be endless. Of course, just because it can detect a lie doesn't mean that it would always detect true statements (since some people may believe what they're saying, even if it's that two plus two is three).
5) Non-fat Donuts - The key is to have the same enjoyable taste without the fat or the calories. An extra bonus if it's actually healthy for you (vitamins included, 2 servings of fruits without the fruity taste or chunks, etc.).
6) Pet Interpreter - Think of it as the Dr. Dolittle of devices. With this voice (er, bark) activated device you would be able to know exactly what your 4-legged friend is saying. Of course, this could pose an issue if you have a guard dog that keeps barking "I'm scared, please, please, please don't come inside" at a thief who happens to have one of these devices.
7) Teleporter - Unlike everything else on this list, this item has been talked about for decades. While it may not be a new idea, the potential of it still requires that it be listed. Imagine teleporting to Paris within 5 minutes before heading to New York City. You could literally see the world in less than a week without having to deal with airlines, baggage fees, or TSA (those reasons alone make this invention a must-have).