Collaborative Mind Mapping
I like Mind Mapping -- basically visual outlining with as many interconnections as you like. I don't love Mind Mapping, but I like it well enough to try out software.
Here are two tools for collaborative Mind Mapping that I'm considering using in upcoming talks:
(Bonus points to Mind 42 for Hitchhiker's Guide to the Galaxy reference.)
My favorite tool is still MindManager on a Tablet PC.