LibreCAD - An open source 2D CAD application for Windows, Apple and Linux.
Sculptura - 3D sculpting for everyone
FreeCAD - An open-source parametric 3D modeler
Nomad Sculpt - Nomad is a sculpting & painting mobile app, available on Android and iOS.
SketchUp - 3D for Everyone
PaintCube - Web-based 3D painting software