Atomic is described as 'Work your way up from a blank canvas, or pull in designs from Photoshop or Sketch. Lay out your states and instantly animate between them, without coding a thing. Share your prototypes with anyone or view on any device via a humble URL'. There are more than 25 alternatives to Atomic, not only websites but also apps for a variety of platforms, including Mac, Windows, iPhone and SaaS. 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 Atomic are Moqups (Freemium), Principle (Paid), Origami Studio (Free) and STUDIO (Freemium).
SavahApp is a collaborative platform that streamlines feedback right to your design screen that allows designers to create high-fidelity prototypes for web and mobile apps without writing a single line of code.
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.