Hourglass - Hourglass is the most advanced simple countdown timer for Windows.
Free Alarm Clock - Free Alarm Clock is a free, full-featured and user-friendly computer alarm clock for Windows.
Alinof Timer - Alinof Timer is a very easy to use timer for your Mac.
Gnome Clocks - Clock application designed for GNOME 3
CookTimer - CookTimer is a simple cross-platform countdown timer that helps when cooking, making tea or eggs.
AlarmDJ - Online Alarm Clock - Use YouTube videos or MP3 files as alarm sound. Works offline.