I have to switch between various machines so I use my phone. Then I find any app to be overkill and limiting at the same time. I recommend using a timer and your phone's timer will do. Set timers for 25mins, 5 mins and 15mins and go!
They are magnetic, start the countdown at the given time (like 25 min and they remember it), the beeper is very soft and you can't procrastinate exploring hundreds of features.