Normally it is the key under the Escape key and left of the '1' key on your keyboard. Activating the console pauses gameplay and allows direct manipulation of Fallout 3 that is just below using an editor. A full list of commands can be found at http://community.livejournal.com/fallout_ru/91089.html
Beware, extensive use of the console can result in visual defects. I recommend creating a separate save game to use the console.