Yeah, they used it to make a hot drink, a sort of unsweetened cocoa. Not much different in effect than hot coffee, another popular drink. To the world, the Americas contributed corn (maize), tomatos, peppers, potatoes, coffee, chocolate, and (on the darker side) tobacco, cocaine, and marijuana.
Hot Choclate quickly became a Eurpean favorite as well. I wonder who had the bright idea to add sugar? Just thinking about it makes me want to go to the kitchen and fix a cup.
Marijuana (C. sativa, C. indica, and C. rubeydis) is native to Asia (and possibly Africa), not the new world.
Other new-world plants include peanuts, pineapples, sweetpotatoes, numerous legumes, cashews, papayas, squashes, and guavas. Cotton appears to have been domesticated both in the old world (short-staple cotton) and new world (long-staple cotton).
Also the strawberries varieties we now grow are a product of two native North American varieties.