Ever in the middle of a presentation and the Mac screen dims or your Mac goes to sleep? Yes, you can go in and edit your preferences to avoid this, or you can get Caffeine.
It's super easy to use and does one thing extremely well:
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. The menu now has a sub-menu for deactivating Caffeine automatically after a number of minutes.
Verdict? It's awesome. Get it now. It's not open source, but it is freeware.