Playscii - Full-featured ASCII art and graphic editor and game creation tool
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
Visio - Simplify and communicate complex information using data-linked diagrams.
yEd - yEd is a free desktop application to quickly create, import, edit, and automatically arrange diagrams. It runs on Windows, Mac OS X, and Unix/Linux.
IOctopus - IOctopus is an online mind map service for a dive deep into the mind for the best ideas.