Golden - Mapping all human knowledge
Dia - Dia is a GTK+ based diagram creation program for GNU/Linux, MacOS X, Unix, and Windows, and is released under the GPL license.
Sutle - Save and organize resources into a directed learning path.
draw.io - Online diagramming application
Self-Defined - A modern dictionary about us.
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.