Zoho Wiki is a free hosted wiki with WYSIWYG editor, versioning, sub pages, tags, dashboard, rss feeds and it is search engine friendly. With Zoho Wiki you can create groups and add, edit and share contents among them. Like all Zoho products it works like a charm so you have to check it out to see all it's features.
Wikispaces is a hosted wiki creation tool. The main goal of the developers was to make the process of creating wikis as simple and effective as possible. 1,000,000 users show they were successful.
WetPaint is a free and very easy to use wiki engine. Create your own public or private wiki and choose your theme from a wide variety of templates. Invite others to edit your wiki and collaborate.