While there are plenty of tools for creating Wikis, Wikispaces happens to have a great interface and provides unlimited ad-free pages for K-12 educational purposes.
It took about two days for me to upgrade a free wiki that I created to the "Plus" account that the company provides to K-12 groups. You can keep developing the wiki before the upgrade is complete; the upgrade itself requires a few clicks and some basic information about you and your educational group. The upgrade, most significantly, gets rid of the ads that support the free wikis the company offers, but also provides more granular control over editing privileges, customizable themes, and enhanced security.
Perhaps I'm getting ahead of myself here, though. While most of us know what a wiki is, that knowledge is certainly not universal among teachers. When I told a group of teachers that we would be using a wiki to align and redevelop our curricula district-wide, I was met with, well, concern. While some had used Wikipedia, no one had ever edited Wikipedia before, much less a more private wiki. As I defined wikis for the uninitiated:
A wiki is a living document or set of documents on the Internet that can be updated by a variety of users concurrently. It provides a unique opportunity for collaboration as all users can create and edit documents.
A quick demo and a promise of training seemed to allay their concerns as it became quite obvious that a wiki was a powerful, easy tool for putting together documents that can be viewed and/or edited from anywhere. The idea, in fact, that they could quickly and easily build documents online and avoid hassles of sharing and assembling documents via email, shared network resources, or, worse yet, sneakernet, was inherently attractive and the benefits are undeniable.
I'm highlighting Wikispaces here partly because of their commitment to K-12 education and the free products they offer to this market. However, the interface is particularly friendly for nervous teachers, too. Join the wiki (usually by a simple click through from an email sent by the wiki "organizer"), click "Edit this page" or "New page", and a simple WYSIWYG editor with solid table integration and an optional text interface pops right up.
It's free, it's easy, and Wikispaces isn't placing a limit on the number of wikis that you can create (although you have to go through the upgrade process described above for each new educational wiki you make). This one's a no-brainer, folks. If you don't already have a wiki or 2 or 10 for various purposes within your district, get one and use it well.