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.
In particular, it is optimised for making theories of change, logframes, etc. – special kinds of pragmatic causal theory which are particularly popular at the moment in the world of monitoring and evaluation (M&E).
Theory Maker is most often used by M&E staff for quickly sketching out project theories and evaluation plans. But scientists, especially social scientists, and policy makers use it too.