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.
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!
PlantUML - PlantUML is an open-source tool that uses simple textual descriptions to draw UML diagrams.
MindNet - MindNet is an effective productivity and learning tool that allows you to take notes, write down ideas, learn, brainstorm your mind, and much more in intuitive graphical mind maps.