Kokuyo Karu Cut Washi Tape Cutter
This washi tape cutter's spring-loaded alligator clip design makes it extremely easy to use. Simply press the end with the spring to open the cutter and clip it onto the roll of tape, then unpeel the tape and tear it off against the metal teeth. The cutter slides smoothly over the washi tape roll, so you can easily reposition it and peel off as much tape as you need. It's also easy to take off, so you can quickly switch between different rolls of tape.
See them in use here, at 1:48.