Mind42 ("for two" — the notion being collaboration) is an online mindmapping tool that manages to capture pretty much the best of all the features of mindmapping. You can create any number of child or sibling nodes, move them around by simply dragging them, attach files and notes to them, use images for node text, and set up chat channels associated with nodes. When you're done, you can share your map with a few or many. Or, as I've done below, embed it in your webpage via an iframe:
You can also export it to Freemind, Mindmanager and RTF (although in the last case the result is not very useful). Mind42 seems as though it might be the best of the free and online mind mapping tools available at the moment.
Mindmapping earlier on Slaw: