LibreCAD - An open source 2D CAD application for Windows, Apple and Linux.
Modelio - Modelio is an open source modeling tool supporting natively UML, BPMN and model exchange with XMI.
FreeCAD - An open-source parametric 3D modeler
Diagramo - Diagramo is an online flowchart software - diagrams (create, edit and share)
SketchUp - 3D for Everyone
UML Designer - UML Designer provides a set of common diagrams to work with UML 2.5 models.