Focusey

Focusey is an incredibly simple to use and effective timer and checklist app for the Pomodoro Technique. It always sits in your menu bar and you can customize it according to your needs. Start the timer using 'Play' button in the app's main window. It will start you first cycle aka Pomodoro.
By default it means that you will work for 25 minutes and take a 5 minute break. After 4 cycles you should take a longer break, by default 20 minutes. The app lets you customize your technique by changing the time of respective parts of the cycle and intervals between long breaks. Focusey also helps you visualize how much time you spent on your task and learn to better estimate the time that you will need in the future buy counting how many cycles each task took.