Wireframe.cc - Sweet and simple online wireframing tool.
Synfig Studio - Just another WordPress site
WireframeSketcher - WireframeSketcher is a wireframing tool for creating wireframes, mockups and prototypes for desktop, web and mobile apps. It's a desktop app for Mac, Windows and Linux, and a plug-in for any Eclipse IDE.
OpenToonz - OpenToonz is an animation program that is a free software version of Toonz, the animation software...
Saola Animate - Easily create engaging HTML5 animations and interactive web content without coding skills
Blender - Blender is the open source, cross platform suite of tools for 3D creation.