PBworks is a commercial real-time collaborative wiki editing system, with HTML editing abilities.

Users can create free basic wiki workspaces, or upgrade to a premium plan to access additional features, such as enhanced security features, customization through CSS, and more storage space. Workspaces can be public or private (only viewable by those who have been invited to join the workspace).

How it works

This video shows how PBworks works:

Alternatives:

Google Docs could be used as a collaborative wiki style system.