Not sure what a wiki is? Watch this video.

Free wikis:
PB Wiki
Wet Paint

50 ways to use wikis in your classroom.
Getting Tricky with Wikis - Tips on how to do cool things to your wiki.

Some wikis teachers have used with their classes:
Pasta Mining
6th grade English
Algebra 1 stats project
Aristotle's poetics
AP world history
Presidential election