Mac: Pomodoro timers are easy enough to find, but if you're looking for something that's incredibly simple and free, Pomodoro One does the job. It's a no-frills app, but that's kind of the point anyway.
Pomodoro One does exactly what you'd expect. It's a Pomodoro timer that has an alarm that goes off after a set interval. Then, it sounds an alarm and gives you a break from work. When the break's over, it automatically starts timing your work again so you can't mess around off the clock. The app also tallies up how many of these cycles you go through during the day.
Pomodoro One (free) [Mac App Store]