Inkscape - Inkscape is a free, open source professional vector graphics editor for Windows, Mac OS X and Linux.
Invision - Prototyping and collaboration for design teams
Blender - Blender is the open source, cross platform suite of tools for 3D creation.
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.
Sculptris - Sculptris: Enter a world of digital art without barriers.
Zeplin - Collaboration app for UI designers & frontend developers