Self-leveling 360 degree Laser Level
One of our most used tools... once you own one of these there's no turning back.
First, it's self-leveling. No squinting, tapping, adjusting. And the green lasers have 100 feet of range so finding a level line across an entire room or even outdoors is no problem.
We've used it for railings and fencing, lining up junction boxes, correcting the level of floor plates, marking places on the ceiling for lighting... basically, it makes common things much easier, and then gives you precision for things that would otherwise be nearly impossible.
Just do it, you won't regret it!