OmniGraffle

OmniGraffle helps you draw beautiful diagrams, family trees, flow charts, org charts, layouts, and (mathematically speaking) any other directed or non-directed graphs. We've had people use Graffle to plan plotlines for a story, make an overview of an operating system, show the evolution of computers, and even show how diseases can spread in a closed population. If you want to organize your thoughts, your projects, or even your friends graphically using boxes and lines, OmniGraffle is your tool.