This is a small application, in both size and functionality, but I have found invaluable.  Using a MacBook Pro one of the things that bothers me is how often and how frequent the monitor dims and the computer goes to sleep.  However, sometimes, while not at the computer I want the computer to fall asleep early.  Going to system preferences everytime is time consuming.  Caffeine solves this problem.  By clicking the application icon in the menu bar it activates, and thus, while activated it keeps the computer awake.  Great for giving presentations, screencasts, watching videos, or any other time you need to keep your Mac awake.

From the developers site:

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.

Download now