SnapTimer - SnapTimer is a simple, free, portable countdown timer for Windows.
TIMEBOX - An advanced timer for professional meeting facilitators
Gnome Clocks - Clock application designed for GNOME 3
Hourglass - Hourglass is the most advanced simple countdown timer for Windows.
Free Countdown Timer - Free Countdown Timer is a free, full-featured and user-friendly countdown timer for Windows
MetaClock.com - MetaClock is an Online Alarm Clock that can play YouTube videos and uploaded MP3s as an alarm tune. It supports Chromecast which means you can turn your TV or stereo system into an alarm clock. No installation is needed, it can also work offline.