Amphetamine

Amphetamine is a little app that lives in your menu bar which can effortlessly override your energy saver settings and keep your Mac awake.

Free Mac OS X

Caffeinated

Caffeinated is a simple utility to prevent your PC from going to sleep. It places an icon near the clock. Click the icon to keep your PC awake. Click it again allow automatic sleep. Caffeinated is heavily inspired by Caffeine for OS X.

Free Open Source Windows

Sleep Preventer

Prevents your computer from going to sleep, hibernate, dim the display or start the screensaver. Application runs in the system tray when closed. New in version v1.3:...

Free Windows