draw.io - Online diagramming application
OpenToonz - OpenToonz is an animation program that is a free software version of Toonz, the animation software...
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.
Synfig Studio - Just another WordPress site
PlantUML - PlantUML is an open-source tool that uses simple textual descriptions to draw UML diagrams.
Blender - Blender is the open source, cross platform suite of tools for 3D creation.