There is a free wikispace for teachers. Teachers can connect with their students and other educators and use their wikispaces to post their lesson plans, create classroom notice boards, etc.
I found the following blog to be very interesting. A first year teacher was up late one night lesson planning when the thought crossed his mind that there were other teachers doing the exact same thing. He created a wikispace to post lesson plans, reproducibles, links and presentations. He hopes that other teachers will create a wikispace so that there will be a virtual library of resources available to educators.