Windows only: Tiny utility Caffeine disables or enables the screensaver by simply double-clicking on the icon in the system tray—so you can watch that YouTube video without your screensaver kicking in.
Using the software is easy enough—just double click on the icon in the system tray to turn the screensaver on or off. The application works by simulating a keypress every minute to fool the system into thinking somebody is sitting in front of it. The application also has a number of command-line options that can be used to automatically terminate after a certain amount of time, which could be useful if you frequently watch movies on your PC but want the screensaver to come on after it's over.
Caffeine is a free download for Windows only. If you'd rather not use a resident application wasting memory all the time, check out how to create shortcuts to actually enable or disable the screensaver. Thanks, Randy!