Draw.io is a free web based diagramming and mind mapping software.
Microsoft Visio (starting at $299.99) is a diagramming and vector graphics application and is part of the Microsoft Office suite.
Cacoo is a cloud-based online diagram tool for creating flowcharts, mind maps, wireframes, and more. It's also an alternative to Visio with Visio import features.
Inkscape is an open source vector graphics editor that among its tools offers solutions for charts, diagrams or schematics via connector-based paths and various uses of LaTeX.