I hate it when I am watching videos on my Mac and the screen dims. I guess watching a video doesn't count as "activity", so the computer starts entering power-saving mode.

Turns out there is a solution to this problem. Caffeine is a small, free application for the Mac that puts an icon in your menu bar. If you click the menu, it prevents your Mac from dimming the screen, going to sleep, or doing other power-saving things. Click it again and it's back to normal. This is a great little application to have for when you are watching videos, listening to music, or using your computer in other ways which don't have you typing or using the mouse.