ProtoShare - ProtoShare is a web-based, collaborative wireframing and prototyping tool.
OpenToonz - OpenToonz is an animation program that is a free software version of Toonz, the animation software...
PowerMockup - Mockup & Wireframe toolkit for PowerPoint
Synfig Studio - Just another WordPress site
Wireframe.cc - Sweet and simple online wireframing tool.
Blender - Blender is the open source, cross platform suite of tools for 3D creation.