Hourglass - Hourglass is the most advanced simple countdown timer for Windows.
TimeLeft - TimeLeft is a versatile desktop utility - it can be used as a countdown clock, reminder, clock...
SnapTimer - SnapTimer is a simple, free, portable countdown timer for Windows.
Smart Sloth - Smart Sloth uses the concept of timeboxing, which is to allocate a set amount of time to each task, therefore, creating a deadline which can increase the productivity of the user.
Be Focused by XWaveSoft - Simple Pomodoro timer in your Mac's menu bar
ChirpyTime - ChirpyTime: Timer / Alarm Clock / Clock app for iPhone / iPod Touch