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