Proto.io is described as 'Create fully-interactive high-fidelity prototypes that look and work exactly like your app should. No coding required. Companies like IDEO, Disney, Paypal, Airbnb and more use Proto.io' and is an website in the Development category. There are more than 25 alternatives to Proto.io, not only websites but also apps for a variety of platforms, including Mac, Windows, SaaS and Linux. The best alternative is Adobe XD. It's not free, so if you're looking for a free alternative, you could try InVision or Marvel. Other great sites and apps similar to Proto.io are Balsamiq Wireframes (Paid), STUDIO (Freemium), Wireflow (Free, Open Source) and Presentator.io (Free, Open Source).
Famous is a design tool that allows you to create and publish live progressive web apps. It's the only tool in the market that lets designers generate a live experience without needing to know any code and without needing any engineers.
Xiffe is an ideal platform for creating interactive prototypes for your mobile apps in a jiffy. It lets you drag-drop native controls for iOS, Android and Windows Phone (and tablet) apps and create interactive prototypes.
Upload your designs,mockups or wireframes and organize them in projects. Share them with friends, colleagues and clients to get point specific Feedback. Giving Design Feedback is easy possible without any registration by your customers. You can Upload Images and Multipage PDFs
Markly is a measure and design spec plugin/extension for Photoshop & Sketch. It is made for modern web designers & app front-end developers. You can add specification marks by simple clicking and dragging.