FreeCAD - An open-source parametric 3D modeler
LucidChart - LucidChart is the missing link in online productivity suites. LucidChart allows users to create, collaborate on, and publish attractive flowcharts and other diagrams from a web browser.
Blender - Blender is the open source, cross platform suite of tools for 3D creation.
draw.io - Online diagramming application
Onshape - Onshape is the first full-cloud 3D CAD system. It runs in a web browser and on any mobile device.
PlantUML - PlantUML is an open-source tool that uses simple textual descriptions to draw UML diagrams.