Caffeine is a very small utility that prevents your Mac from going to sleep. You have to love the name of this app considering what it does. The app does three things: it prevents your Mac from automatically going to sleep, dimming your screen or running a screensaver. This is an excellent app for when you want to watch long videos and don't want your Mac to dim your screen or don't want to be bothered with screensavers. One thing it does not do, however, is to prevent your Mac from going to sleep when you close your Macbook's lid. This is the most common complaint about this application. I remember using InsomniaX to do that on my non-pro Macbook, but InsomniaX doesn't seem to be regularly updated any more.
Caffeine sits on the menu bar, and you can change its status by clicking the icon. The icon is a cup of coffee. When it is activated, the cup is filled with coffee, and it is empty, when the app is inactive. Right-clicking the icon will launch a menu from which you can access the about window, preferences, and set an activation time limit.