Create your own visually-customizable wiki, allow other users to add content to your wiki on a particular subject, with features that allow you to message and communicate with other wiki creators on the site.

Oldest but most basic offering for running a wiki. Free if the wiki is public, and ad-supported. Nominal charges if you wish to make it private or eliminate ads. From their site: " What can I use Wikispaces for? Wikispaces is great for any kind of group website. It's for families, classrooms, sports teams, community groups, book clubs, fan clubs, party organizers, wedding planners, and more."