Build Walls With Laser Lines
Just line up the laser where you want the wall to be, and you'll see lines on the ceiling and floor. You can leave the laser in place and build to the laser lines, or you can use the lines to make marks for the top and bottom wall plates. Either way, it's much faster and easier with a laser. A self-leveling cross-line laser works great for this. You can buy one at home centers or online.
Find the High and Low Spots in a Floor
When you start a kitchen cabinet installation or tile a wall, you need to know whether the floor is level and how much variation there is from the high and low spots. It's easy to figure out with a laser level.
Just set the laser on a few scraps of wood or a paint can in a spot that will project a beam across the floor. Make a quick sketch of the floor plan. Then pick a spot and extend a tape measure to the floor. Note the measurement where the laser beam crosses the tape, and mark this on your sketch. Then check other areas of the floor and jot down these measurements. The location with the smallest dimension is the highest spot, and the difference between the largest and the smallest dimensions tells you how much the floor is out of level.