draw.io - Online diagramming application
RemoteHQ - Real-time collaboration for distributed teams
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.
Terrastruct - A diagramming tool for software architecture
PlantUML - PlantUML is an open-source tool that uses simple textual descriptions to draw UML diagrams.
Diagrams for Mac - A diagram editor for visualizing structures and processes