Wikispaces

Wikispaces is one of the major wiki hosts (claims to have over 1.5 million wikis and 10 million users) with a mix of free and paid plans, and a focus on K-12 education. The site will close down in 2018 (on July 31 for free wikis, at the end of the year for paid ones), leaving most of those users scrambling for a new host to migrate to.