Other tools worthy of consideration (approximate decreasing order of server load/bandwidth requirements):
OpenSim (Second Life clone) http://opensimulator.org/wiki/Main_Page
OSTube (German YouTube clone)
DimDim (live audio and video conferencing, including the dreaded PowerPoint capability)
Mahara (easily build individual sites with rich media content)
Moodle (very, very good course management system - perfect for delivery of formal online courses. Even includes PayPal module)
WordPress Multiuser (already in use here on the Pub but could probably use an upgrade).
I don't know these, yet, but will be interested.
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