Mac: Chances are you have at least a few programs that you don't want to go to sleep once you've opened them. Wimoweh is a menubar app that lets you designate certain programs to keep your 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 certain background tasks.