Dracula Graph Library!

Graphviz is open source graph visualization software. It has several main graph layout programs. It also has web and interactive graphical interfaces, and auxiliary tools, libraries, and language bindings.

Theory Maker is a free web app by Steve Powell for making any kind of causal diagram, i.e. a diagram which uses arrows to say what contributes to what. You make the diagrams just by typing the names of the elements (called variables) in a structured way into a (resizeable) window, and you get a live diagram as output which reflects what you type.