When it comes to iconic fast-food chains, few can rival the cult following of In-N-Out Burger. Founded in 1948, this California-based burger joint has gained legendary status, not just for its simple yet delicious menu but also for its not-so-secret "secret menu." While the official menu boasts classics like the Double-Double and hand-cut fries, those in the know can unlock a world of customizable options that elevate the In-N-Out experience.