Graphviz is a graphing toolkit that allows you to render several sorts of directed and undirected graphs like this:
Pixelglow has a very neat Mac OS X port of Graphviz, complete with a new document-based GUI, export to PDF and many more bitmap formats, full alpha transparency, native font support and anti-aliasing:
The dynamic zoom has to be seen to be believed, and the properties sidebar is a great way to tweak the layout. All in all, a great example of how “old school” UNIX tools can be adapted to run under Cocoa.
There is also an amazing iPhone implementation called “Instaviz”:AppStore:Instaviz.