Windows: If you have a quick-launching screensaver or a have your laptop's screen dim itself on battery power, reading and watching videos can get annoying. Free app Mouse Jiggler keeps your computer awake so you don't have to worry about it.
Most modern video players will disable the screensaver automatically, but a few have yet to gain this feature (I'm talking to you, HTML5-powered YouTube). Furthermore, if you're trying to do some reading, it can get annoying to have to jiggle the mouse yourself every few minutes to stop your screen saver or un-dim your laptop's monitor. ScreenJiggler is a tiny program that runs on your desktop and subtly jiggles your mouse to keep your computer awake.
You can also turn on "Zen Jiggle", which will simulate mouse jiggles without actually moving the cursor. The app's window doesn't minimise or hide itself either, so you're never confused about when it's on or off — you don't want to leave the house with your screen saver still disabled.
Mouse Jiggler is a free download for Windows only. Mac users, check out Jiggler for similar functionality.