Automatic diagram generation with HTML output

Does any one know of a good, open source, diagram package/library that
generates html pages?

I need to show a complex relationship between web pages, a normal tree
structure would not be enough.

IsBOF

OmniGraffle from the OmniGroup is a great diagraming tool, though
it's not open source. It outputs to HTML and handles linking in the
html output as well. I use it for diagramming classes as well as
quick & dirty application flow mockups.

Niels

GraphViz is good for mechanical generation of diagrams of
directed graphs. It generates a variety of image formats,
as well as image maps. These can be folded into web pages.

-r

here it is, and yes, it's foss:
http://railroad.rubyforge.org/