Libre office draw is wonderful, it can edit pdf docs at ease. It’s crazly powerful compared to paint 3d
Pingboard - Pingboard is the employee experience suite for growing companies that put their people first.
draw.io - Online diagramming application
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.
yEd - yEd is a free desktop application to quickly create, import, edit, and automatically arrange diagrams. It runs on Windows, Mac OS X, and Unix/Linux.
Visio - Simplify and communicate complex information using data-linked diagrams.
PlantUML - PlantUML is an open-source tool that uses simple textual descriptions to draw UML diagrams.