If you use a laptop then you've probably experienced having to shake your mouse every so often just so that the screen doesn't dim while you're watching a video. That can be really annoying at times, especially on YouTube.
Caffeine was designed just to solve that. It's Linux only for now.
To install it, you can simply download a debian package from its launchpad page, or if you automatically want the latest updates then you can add the repository to your apt-sources.
In Ubuntu, simply enter these commands from your terminal:
You can then access Caffeine from Applications -> Accessories -> Caffeine.
Don't forget to enable its "Activate for flash video" setting! This lets it automatically detect when you're watching flash video online and auto-enables Caffeine.