Free Alarm Clock - Free Alarm Clock is a free, full-featured and user-friendly computer alarm clock for Windows.
Gnome Clocks - Clock application designed for GNOME 3
AlarmDJ - Online Alarm Clock - Use YouTube videos or MP3 files as alarm sound. Works offline.
Alarm Clock Xtreme - Alarm Clock Xtreme: Alarm, Stopwatch, Timer (Free) is the most popular alarm clock app created and published by AVG Labs for both iOS and Android devices.
Alarm Clock (applet) - Alarm Clock is a fully-featured alarm clock for your GNOME panel or equivalent.
KTimer - KTimer is a little tool to execute programs after some time.