Xmind - Xmind is a brainstorming and mind mapping application.
draw.io - Online diagramming application
Coggle - Coggle is a simple, beautiful, powerful way of structuring information.
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.
Mindomo - Easy-to-create and share mind maps, concept maps, task maps and outlines. Mind mapping software for Web, Desktop, iOS and Android. Mind map with us for free!
PlantUML - PlantUML is an open-source tool that uses simple textual descriptions to draw UML diagrams.