Gliffy looks extremely cool; it’s intuitive and smart, and my first impression was that this would be a genuinely useful app.
Gliffy is an online diagram-creation application. It has shapes for flowcharts, floorplans, networking, even UI design. You can print your diagram directly, or save it as a SVG, JPG, or PNG file — pretty good options, I’d say. Below is a test diagram I made in just a couple minutes…

My hesitation is that it is built entirely in Flash; the Linux flashplayer, I’ve found, frequently breaks complex flash applications/designs. (The Doom3 web page was a famous example). I’ll be able to test Gliffy on Linux later; hopefully I’ll be pleasantly surprised. If it works, then I don’t have a problem with it; I’m not one who is very worried about software being RMS compliant.
Check out gliffy; if you ever need to make diagrams at all, it looks like a handy (free) tool!

I’ve been using CmapTools for that sort of thing. http://cmap.ihmc.us/ It works pretty well. If all you need is a tree Freemind works well. http://freemind.sourceforge.net/wiki/index.php/Main_Page
Thanks for the pointer. I’ll check it out.
Thanks for the recommendations; both of those pieces of software look interesting.
They seem to serve a little different goal than gliffy, which also has the advantage (possibly drawback, depending on your point of view) of being web-based; but they both seem pretty cool.