Wikis

Wikis are a collection of easy to edit web pages. When you create a wiki, you can control who can see it and who can edit it.You can also track any changes, seeing when they were made and by whom.

Wikispaces offers K-12 educators free, ad-free wikis for educational use. You can also add bulk student accounts even if the students do not have email addresses.
Get started at http://www.wikispaces.com/t/x/teach/tho2010
Wkispaces' Teacher Help page offers lots of support and explains how to add different resources to your wiki pages.

Educational examples to browse:

Supporting English Learners

Instructional Support

Community College Composition class

High school

Welker's Wikinomics offers resources for students and teachers of economics.
Applied Math 40S Wiki Solutions Manual offers a space for students to share solutions to math problems.
Elizabethan Period is a pathfinder created by a high school librarian to support student research. (Most of the databases on top require an account, but the other resources further down the page are free websites.)
Search Tools Wiki - also by Joyce Valenza

Middle school

Mr Bergmann's 8th grade Science class
Ah Bon French offers French beginners in middle school notes, flashcards, videos, and online exercises to use at home for extra practice.
In the Flat Classroom Project, middle school and high school students from schools around the world collaborate on digital projects. They use additional wikis to organize and post the student projects.

Elementary School

Flat Stanley's Wiki for Stacy Colwell's first grade class at Sutter's Mill School. Each time Stanley visited a new location, the info was added to the wiki page.
At Salute to Seuss, participating elementary schools used wiki pages to post student work related to one book by Dr. Seuss.
Littlewood's Nature Guide for Littlewood Elementary School's Environmental Center was created by a fifth grade class.

Share Lesson Plans

Curriki , a play on the words 'curriculum' and 'wiki', is a nonprofit organization started by Sun Microsystems that is using wiki software to develop an online repository for K-12 curricula and instructional materials in mathematics, science, technology, reading and language arts, and languages.

Browse an annotated list of more educational wikis.

Introductory video - Wikis in Plain English from Common Craft.