I watch a lot of videos on various sites including Youtube, Google video, etc., and one of my main gripes with the sites is that video does not make your mac think it is busy so it constantly falls asleep while playing a single clip.
In comes the freeware application Caffeine.
Caffeine is a tiny program that puts an icon in the right side of your menu bar. Click it to prevent your Mac from automatically going to sleep, dimming the screen or starting screen savers. Click it again to go back. Hold down the Command key while clicking to show the menu.
Caffeine is one of my must have applications and can be downloaded here.