Caffeine, stops your Mac going to sleep! Genuinely useful, lean and simple to use. It even has a cute icon

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.
Free for Mac OS X 10.4 or later.