Hint: See list of open source diagram software at bottom of https://en.wikipedia.org/wiki/CmapTools
Diagramming Software draws mind maps, argument maps, graphs, UML diagrams, flow charts, etc.
Cloud Collaborative Embeddable Options
- Open License options - 
- Dia Server?
- Open source or free - 
- Any cloud collaborative versions available?
Helps create knowledge models represented as concept maps.
- mac only
Graph visualization is a way of representing structural information as diagrams of abstract graphs and networks. Automatic graph drawing has many important applications in software engineering, database and web design, networking, and in visual interfaces for many other domains.
Graphviz is open source graph visualization software. It has several main graph layout programs. See the gallery for some sample layouts. It also has web and interactive graphical interfaces, and auxiliary tools, libraries, and language bindings.
Online 3D mindmapping
2D vector graphics editor. Has all standard drawing features. Curves, lines, freehand drawing, gradients etc. Includes bitmap tracing support.
Tool that creates different kinds of diagrams. It currently has special objects to help draw entity relationship diagrams, UML diagrams, flow charts, network diagrams, and simple circuits. It is also possible to add support for new shapes by writing simple XML files, using a subset of SVG to draw the shape.
You can define part libraries using Dia for quick drag and drop of symbols into a document, but that is done using XML. 
The goal of the StarUML project is to build a software modeling tool and also platform that is a compelling replacement of commercial UML tools such as Rational Rose, Together and so on.
Open-Source GUI Prototyping Software, but is also useful for creating charts of all kinds. Commercial license also available for modifying source code.