An instructor and editor must be assigned to carefully monitor and read posted material. Inappropriate material must be quickly removed, while meaningful comments need to be archived and prepared. POTS Not the green leafy stuff, but the plain old telephone system. Yes, one of the greatest collaboration techniques is to integrate teleconferencing into your web-based training. They'll get a case study and discussion questions to download and essentially talk over the phones.
Several vendors sell proprietary systems designed to facilitate collaboration.
Look for a system that supports all the collaboration features you need. For example, Databeam's technology allows you to demonstrate software applications then pass control to participants, where most other systems do not. Placeware has a great small group feature that allows the instructor to break the group into small teams like you might do in classroom training. I do not feel that motion visuals are necessary to communicate between participants so steer away from products that push sizzle over substance. Again, my experience indicates that most of the time participants turn off the motion and focus on the content of the communications when using these systems.