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.
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.
Bizagi Modeler - Business Process Modeling Software. Bizagi BPM software puts simplicity first.
OmniGraffle - OmniGraffle is for creating precise graphics like website wireframes, an electrical system designs, or mapping out software class.