Synfig Studio - Just another WordPress site
Invision - Prototyping and collaboration for design teams
Animation Paper - Animation Paper is a software package for doing 2D animation.
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.
CrazyTalk Animator - CrazyTalk Animator, the next level of 2D animation. Create actors using any photo or illustration.
Balsamiq - Balsamiq. Rapid, effective and fun wireframing software.