Blender - Blender is the open source, cross platform suite of tools for 3D creation.
Flowkit - Sketch library for user flows/content maps/annotations
SketchUp - 3D for Everyone
Crystal: Sketch Mirror for Android - Crystal: Sketch Mirror for Android is an application that is designed for viewing your sketch designs in real-time, navigate prototypes and download content related to the sketches for offline viewing.
AC3D - AC3D is a 3D software for games, virtual reality and flight simulation, scientific, medical and general data visualisation, rapid prototypes of 3D designs, 3D printing, Google Earth, X-Plane, Second Life and more.
Sketch Mirror - Preview your iOS designs directly on your devices