Looking for wireframing and site map tools

I’ve been looking for tools to help with site maps and wireframing.  Visio is not the answer here.  I’m much more interested in the functional architecture of the page and site than pretty layouts.

In no particular order, here’s what I’ve found so far:

A few months ago I stumbled across something that looked great (and was open source) but now I can’t remember.  This is definitely an ongoing research project.  Please comment with other ideas.