Non-Linux FOSS: Caffeine!
Okay, this program is free (beer), but not Free (speech). I wouldn't normally include a freeware application in a "Non-Linux FOSS" piece, because quite frankly, it isn't FOSS. But, I decided to break the rules a bit here because I realized how often I use a freeware program when I'm on OS X that I couldn't imagine doing without.
If you use OS X for presentations or demonstrations, you've probably had your screen shut off while you explained a slide or dialog box. Then the screen probably locked, and you had to hurry over to the keyboard so you could unlock, and so on and so on. Caffeine is a simple app that does nothing more than keep your Macintosh (or Hackintosh) computer awake. It runs as a cute little icon in your menu bar, and it disables screen savers and sleep mode, even if you have aggressive power-saving settings enabled.
The danger is that you could leave Caffeine running accidentally and completely drain your battery. I've had that happen only one time, however, and I learned quickly to take my computer off Caffeine when I was done presenting. Even with that risk, I find it's worth it to no longer need to wiggle the mouse pointer every minute to make sure my laptop doesn't fall asleep!
Caffeine is available for free in the App Store, or you can get it from its website.