WikiSpaces is a free WikiFarm at http://wikispaces.org.

Features include 
* true Wysiwyg or plain wikitext editor
* visual history of page changes
* rss feeds for pages, spaces, and messages
* unlimited pages and storage at spacename.wikispaces.org
* anonymous page editing allowed
* content licensed under a CreativeCommons license 

Feedback welcome at help at wikispaces dot org.


Remark (Oct 30, 2012): It is no longer the case that "Registered members can create new spaces (e.g. spacename.wikispaces.org) that are unrestricted public wikis." nor is "anonymous page editing allowed".  Currently, only logged in users are allowed to access wikis, and only up to 5 are allowed for free.