Some people like to plan their day on hour-by-hour basis. It helps them stay disciplined and consistent. One of the time-managment methods is called 'Pomodoro Technique' and its main principle is dividing your time into periods of work and short periods of break/rest. My approach is to work for 25 minutes and then have 5-minute break. To keep track of time left to next break a kitchen timer can be used (the timer is often of tomato shape- hence method's name), but I decided to build a device to help me track the time. I don't use it anymore as I built improved version of the timer, but I used version 1 for a pretty long time.
The device is really simple: just a microcontroller (ATtiny13), 5 LEDs to show time left - controlled by 75HC595 shift register (one LED in work state means 5 minutes and 1 minute in break state), 2 LEDs to show current 'state' (that is work (red) or break (green)), a button to pause the timer and a buzzer. It's powered by small one-cell LiPo battery. The circuit was assembled on a perfboard.
The code was written in C (as usual) and it is ugly. I created it back when I had very little knowledge about C or programming in general (i.e. clean code). I used Timer0 to create 1Hz time base to keep track of time. I wrote a few basic functions to generate sound (emitted on work->break and break->work transitions) and to control the LEDs.