Sleep Cycle - Bio-alarm clock that wakes you up at optimal times based on your sleep patterns.
Free Alarm Clock - Free Alarm Clock is a free, full-featured and user-friendly computer alarm clock for Windows.
Sleep as Android - Common alarm clocks ignore your sleep cycles, they wake you on schedule no matter your are currently in your deep sleep. Waking up from deep sleep is unpleasant, it makes you tired and it may negatively affect your productivity during the whole day.
Gnome Clocks - Clock application designed for GNOME 3
Pillow for iOS - An iOS companion that helps you sleep better 💤
Digital Clock 4 - Beautiful customizable clock with plugins