The list of "things" include:

All of the citrus fruits. (Obviously)It is strongest in lemon and limes though. The more sour the fruit, the higher the Citric Acid content is.

It is found in berries such as Strawberries, Raspberries, and Cranberries.

Exotic fruit like Pineapple and Tamarind contain Citric Acid.

A small amount is found in Cherries.

Citric Acid is also found in some Vegetables! Like tomatoes, Cayenne Peppers, and lettuce!

It improves the clarity of wine, and helps fermentation.

It's also in cheese. Citric Acid is used in the manufacturing process to help clot the milk faster.

Citric Acid is even found in things like Pop! It acts as a flavoring agent.

It works to chelate metals and is useful in laundry detergents and soaps. It works in a way that the cleaners produce foam and helps it work better without needing water to soften it.