Invision - Prototyping and collaboration for design teams
OpenToonz - OpenToonz is an animation program that is a free software version of Toonz, the animation software...
Proto.io - Create fully-interactive high-fidelity prototypes in minutes that look and work exactly like your app should. Sign up for a free 15-day trial and get started.
Synfig Studio - Just another WordPress site
Zeplin - Collaboration app for UI designers & frontend developers
Adobe Animate - Adobe Animate is a Flash, vector animation software.