Mac: There are plenty of Pomodoro timers to choose from but I like apps that disappear when you don't need them -- and are a single click away when you do. With that in mind, Pomodoro Time lets you time your tasks and manage your to-do list right in your menu bar. The Pomodoro technique is a simple method of focusing on your tasks for short bursts; typically you set a timer and work for 25 minutes without distraction and then take a break. As such, Pomodoro Time works exactly as you would expect: You set the timer for your preferred interval and let it run while you work, and at the end you'll get an alarm to stop working. It also incorporates a simple to-do list to accompany your Pomodoro bursts, and you can also designate the length of breaks you want to take between your tasks. It has an optional chronometer sound effect too -- like a ticking clock in the background -- which you may either find amusing or anxiety-inducing.
There is a small banner ad in the free version, but you can pay $US4.99 ($7) to upgrade to Pro.