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.
SketchUp - 3D for Everyone
draw.io - Online diagramming application
Arena - Chess GUI Arena
PlantUML - PlantUML is an open-source tool that uses simple textual descriptions to draw UML diagrams.
PTC Creo Sketch - Creo Sketch is an easy to use 2D CAD application for capturing and creating new 2D freehand concepts which is available on PCs, Macs, and iPads.