Here are some wikis that you might want to try:

Wikispaces: www.wikispaces.com
PBWorks: http://pbworks.com/
Google Sites: www.google.com/ - click the More drop down menu and then Sites

Wikis in Plain English

What is a wiki? Watch this video to find out.
http://www.youtube.com/watch?v=-dnL00TdmLY