draw.io - Online diagramming application
VectorScribe - The missing toolbox for Adobe Illustrator.
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.
Fabricating Alternatives - AI-powered card deck to help ideation & brainstorm
PlantUML - PlantUML is an open-source tool that uses simple textual descriptions to draw UML diagrams.
SnapUp - Wireframe faster / create amazing user interfaces