Caffeine is a tiny open-source program that puts an icon on the right side of your menubar. Click it to prevent your Mac from automatically going to sleep, dimming the screen, or starting screen savers.
Tomas Franzén of Lighthead Software originally developed Caffeine in 2006, and it has been a well-known and loved utility for Mac users for many years. Its simplicity has allowed it to continue working perfectly long after active development had ceased.