I was searching around for a node & arc editor for the Macintosh recently and settled on yEd graph editor by yWorks. While it is mostly a showcase for yWork's graph toolkits it is a very competent editor with good usability. Its visual design also fits in well with the Macintosh. I did try OmniGraffle and its kin because of their broad support for more graphic notations but I found that so much of the user interface emphasis on aesthetics -- pixel alignment, shadowing, etc -- took away from simply getting the diagram drawn. I suspect this observation says more about me than the tool! Anyway, I recommend yEd.