Hourglass - Hourglass is the most advanced simple countdown timer for Windows.
Alinof Timer - Alinof Timer is a very easy to use timer for your Mac.
Horo - The best free timer app for Mac
CookTimer - CookTimer is a simple cross-platform countdown timer that helps when cooking, making tea or eggs.
wnr - Better than pomodoro, this timer app balances work and rest.
RescueTime - Time management software that shows you how you spend your time & provides tools to help you be more productive.