Mobile app design is crucial for mobile app development, as it directly impacts user engagement, conversion rates, and revenues. Various tools are available to assist designers in creating prototypes, wireframes, and mockups. Some popular options include Sketch, FreeHand by InVision, Adobe XD, UXPin, Marvel, Figma, Proto.io, Origami Studio, Axure, Framer, Justinmind, Fluid UI, OmniGraffle, Mockplus, Balsamiq, and Instabug. Each tool offers unique features, integrations, pricing models, and user bases, making it essential to choose the right one for specific needs.