TiddlyMap - An interactive concept- and mind-mapping plugin for the personal note-taking software TiddlyWiki based on the Vis.js library. It's free and open-source!
draw.io - Online diagramming application
Minzel - Minzel is a beautiful and free mindmapping tool built for creatives and gives you the power to...
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.
Wisdomap - Wisdomap Medicine is a study aid that helps medical students create differential diagnoses and suitable management plans from a presenting complaint and structured revision notes.
PlantUML - PlantUML is an open-source tool that uses simple textual descriptions to draw UML diagrams.