Mac: Sleep No More is a small Mac app that sits in your menu bar and keeps your computer from falling asleep for a specified amount of time.
Much like the previously mentioned Caffeine, Sleep No More sits idly in your menu bar, and when you activate it the app prevents your computer from falling asleep. The main difference between the two is that Sleep No More allows you to set a timer for any amount of time you want. This means you can keep you computer awake for a set duration of time when you're downloading large files, transferring data over a network, or for anything else without having to worry about anything getting cut off.
Sleep No More (free) | via One Thing Well