Xmind - Xmind is a brainstorming and mind mapping application.
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.
MindMeister - Create, share and collaboratively work on mind maps with MindMeister, the leading online mind mapping software. Includes apps for iPhone, iPad and Android.
draw.io - Online diagramming application
Coggle - Coggle is a simple, beautiful, powerful way of structuring information.
PlantUML - PlantUML is an open-source tool that uses simple textual descriptions to draw UML diagrams.