Mac: Chances are you have at least a few programs that when you're running them, you never want your computer to sleep. Wimoweh is a menubar app that lets you designate certain programs to keep you computer awake all the time.
Unlike an app like Caffeine, Wimoweh lets you set certain apps that keep your Mac from falling asleep. If they're not running, you computer behaves like normal. If they are, your computer stays awake. It's simple, but incredibly helpful if you need to keep your computer to stay awake for background tasks with certain apps.
Wimoweh | via Addictive Tips