Witeboard - A real-time whiteboard for your team. No signups required.

Diagrams for Mac - A diagram editor for visualizing structures and processes

Whiteboard Fox - Simple Online Whiteboard

Diagrammm - Fundamentals on data visualization

Stormboard - Stormboard empowers data-driven companies to turn their unstructured whiteboards into data-rich collaborative workspaces; enabling data-driven decisions and efficient processes — often eliminating the need for meetings entirely.

Screenshot Flow - Super fast app diagrams to communicate design