Caffeine is a free app that keeps the Mac awake.
Whenever the display goes to sleep on my MacBook, the drive shuts down and stops the back-up. This 2011 MacBook likes to meet energy guidelines and sleeps a lot, even with settings tweaked to the max in System prefs.
- – Dan Stewart (Big Dan)
Also, a Mac laptop may want to sleep when it's not plugged in, but you don't want it to do this while it is showing a presentation.
I have used the free app Caffeine for years and it works great.
- – Keith Hamilton
Once it is installed, click once in the menu bar to tell the computer not to sleep or show the screen saver.
- Caffeine, Mac App Store Preview
- Lightheadsw.com, developer's site (includes an older version for OS X 10.4)
Contributed on Mac-Ministry List in January 2012. Expanded in March 2014.