To no one's surprise, those clear plastic protractors we were all taught to use in primary school are far from being the most accurate way to measure an angle. They're perfect for a year five homework assignment, but in the real world you'll probably want to opt for this digital alternative that's precise to plus or minus 0.3 degrees.
For US$20 it's cheap enough for professionals, students, and anyone who likes to putz around with random contraptions on their desk -- and on a single cell-sized battery you can expect to get about 3,000 hours of use out of it. For anyone not intending to use it as a toy, a locking mechanism keep the rulers at the angle of your choice when drawing lines, while a set of holes at either end let you hang or anchor it in place for tricky measurements. It almost makes me want to go back to school again. (Not in a thousand years.)