A Ning that I have been a member of and find extremely useful and interesting is the University at Buffalo's English Education Ning. There are discussion boards that involve my content area as well as what is going on in the world of education, people share job openings, and you can network with current and past students of UB's English education program. One of the best things about this Ning is that it is constantly being updated and added to, it is never stagnant. I have used this NING as a way to see what others are doing in the classroom, as well as look for job openings.
The only problem I now have with NING is that it has gone to a pay model. There is a sponsor that will pay for your educational NING to stay active, but I have heard talk that the guidelines to qualify are a bit stringent.