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.
Digital Clock 4 - Beautiful customizable clock with plugins
Online Alarm Clock - The world's original online alarm clock, established in 2006.
Alarm Clock (applet) - Alarm Clock is a fully-featured alarm clock for your GNOME panel or equivalent.