TimeLeft - TimeLeft is a versatile desktop utility - it can be used as a countdown clock, reminder, clock...
Hourglass - Hourglass is the most advanced simple 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.
wnr - Better than pomodoro, this timer app balances work and rest.
ChirpyTime - ChirpyTime: Timer / Alarm Clock / Clock app for iPhone / iPod Touch
Timer Applet - Timer Applet is a countdown timer applet for the GNOME panel.