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.
AlarmDJ - Online Alarm Clock - Use YouTube videos or MP3 files as alarm sound. Works offline.
Galarm - Social alarm clock and reminders
Digital Clock 4 - Beautiful customizable clock with plugins
Gnome Clocks - Clock application designed for GNOME 3