Hourglass - Hourglass is the most advanced simple countdown timer for Windows.

wnr - Better than pomodoro, this timer app balances work and rest.

Online Alarm Kur - Free online alarm clock, set alarm easily on your computer using the internet alarm clock without downloading any software, wake up in time with the loud online alarm clock.

Gestimer - For those little reminders during the day

KukuKlok - Kukuklok is a free alarm clock available online and offline.

Floating Timer - Features both a countdown timer and stopwatch that will float over other apps.