Web 2.0 Guru

http://web20guru.wikispaces.com/

One of the best Web 2.0 sites, a lot of information here!

PBwiki

http://pbworks.com/

https://plans.pbworks.com/academic (teacher options, 1 free)

Another option for free Wikis. Use the second link for the free teacher version.

Mediawiki

http://www.mediawiki.org/wiki/MediaWiki

MediaWiki is a free software wiki package. This is for someone wanting to run their own wiki server.