Interactive mockups

Popular options

Adobe Experience Design
Marvel - Mobile-oriented prototyping
Invision - Rapid prototyping for screen (limited free account)
Principle for Mac - rapid prototyping tool for screen UIs, with an emphasis on novel interactivity (macOS only)

Also consider

Atomic - App prototyping with good animation support, conditional behaviour and data-driven interfaces (Web-based, with collaboration)
CanvasFlip - Prototype basic interactivity with timers, interaction flows and gestures. Also includes tools for gathering data from prototypes.
Everybo - Free prototyping and usability testing (web-based)
Fluid - Widget toolkits for mobile platforms, gestures, collaboration and feedback capabilities
Figma - Collaborative UX designing
Framer Modern approach to prototyping motion-rich UIs (macOS-only)
Fuse - Develop full mobile apps with minimal coding
Flinto - Prototype mobile, web or desktop apps (macOS & web-based versions)
Mockplus - Free native app for various platforms
Noodl - Extend prototypes outside of the screen using a visual programming approach, with detailed control over interactivity
Origami - mobile app prototyping, live testing on Android and iOS (macOS only)
ProtoPie - Cross-platform, good support for interactivity and can also utilise device sensors and cross-device communication
Proto.io
UXPin

Functional