Pomodoro timer
HTML UI
The Pomodoro technique is a time management technique that was named after the tomato shaped timer the inventor used. Basically it requires that you work on things for some fixed time (often 25 minutes) and then take a time break (5-10 minutes) and after every four rounds of this take a longer break (20-30 minutes).
Obviously keepnig on this schedule would be easier if you had a timer app that understood it. There are many out there but why not write your own?