Based in New York City, Open Educator, Inc. develops and supports open-source knowledge-sharing communities that make it possible for every motivated K-12 teacher to be highly effective.
Open Planner is Open Educator's free, easy-to-use, on-line planning community.
Registered members can
- author and maintain "open-source" unit plans for review and use by a community of colleagues
- peruse, use, and learn from the planning work of others
- start or join collaborative curriculum authoring teams.
As full-time public school teachers and part-time Drupallers, we're looking for collaborators in the Drupal development community who know more than us. Help us figure out how to tweak this beautiful app to get great things to happen for the K-12 education world.
Please contact me via my profile if you are interested in serving as a technical advisor or board member to our startup non-profit!
Cheers,
Andrew Stillman
Physics Teacher
Co-Founder, Open Educator

Comments
Take a look at wikibooks.org
It looks like wikibooks.org can also accomplish what you want. Difference: there is a group of admins that oversee and can delete books that don't adhere. Good thing, though, is all the scripts, wikicommons images, and templates (chapters, paging, creating info boxes, etc) already available on wikibooks.
I'm using it to create curriculum for k-5 in reading, science, and math (for what it's worth).