What is the most widely used chemical leavening agent?

Baking powder is the most commonly used chemical leavening agent.

Baking powder is a dry chemical leavening agent used to increase the volume and lighten the texture of baked goods. Baking powder works by release carbon dioxide gas into the batter or dough through acid base reaction, causing bubbles in the wet mixture to expand and therefore leavening the mixture. It is used instead of yeast where fermentation flavors may be undesirable or where the batter lacks the elastic structure to hold gas bubbles for more than a few minutes.

Most commercially available baking powders are made up of an alkaline component (typically baking soda, known as sodium bicarbonate), one or more acid salts, and an inert starch (cornstarch in most cases, though potato starch may also be used)