A server, running some kind of WikiEngine where you can set up your own WikiForum without having to clone/program/host it yourself. See also WikiHive.
See WikiFarms for a list of available farms in the world!
Note: If you want to start a WikiWiki on a certain subject, you have to start with a group of people who want to discuss this subject in a WikiWiki. Else you might as well make your own Website with a guestbook. You could start with a wiki and then find the group; it needn't be the other way around. MontanaRowe