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.
wnr - Better than pomodoro, this timer app balances work and rest.
CookTimer - CookTimer is a simple cross-platform countdown timer that helps when cooking, making tea or eggs.
Date Reminder - Date Reminder reminds you of recurring or nonrecurring events, like birthdays, bills to pay...
Gestimer - For those little reminders during the day