draw.io - Online diagramming application
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.
Dot Chart - Dot Chart is simple and quick online process mapping system.
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.
LibreOffice - Draw - Draw, LibreOffice, ODF, open standards, creative software, vector drawing