TimerDoro is a timer app that doesn't do too much or too little. Setup is dead simple because when it comes to productivity, the smallest unit of time is five minutes.
If you use a popular 5-minute time management technique that's related to tomatoes (which I cannot name, lest I be hit with a cease & desist), or if you need the simplest timer available for time management, this is perfect for you.
As the timer ticks down, the background morphs from green to red. If you get sidetracked for a few minutes, you can add more time while the timer is still running, or pause the timer and resume when you're ready.
When time is up, it plays an alert sound. If your app isn't running, it sends a background notification to the Notification Center.