If there's a food that people think is American, it's the hamburger. The original hamburger probably came from German immigrants in the late 1800s, and the first mass-produced modern hamburgers started selling in the 1920s. Some estimates say that hamburgers account for nearly 60% of all sandwiches eaten in the US (and about 7.5% of all our potatoes go towards making French fries). If you like a good burger, check out some of these links.
If you'd like to read more awesome and interesting stuff, check out this unrelated (but not entirely random!) Techdirt post