Freemind is a great mind map tool, but it is not a concept map tool. The Freemind wiki explains, "Unlike mind maps, concept maps are general graphs rather than trees."
If we want a mind map tool (i.e. concept tree) then Freemind is a good starting point. I tried before to rip a bean out of it and it wasn't trivial, so I gave up. There is an applet version, but that is for viewing only.
If we want a concept map tool (i.e. concept graph) then one of these may be the right starting point:
- CmapTools - drawback: requires database backend
- Conzilla - bonus: stores everything in RDF
- Compendium - may be too big