Teachability is a teacher community for sharing ideas, tips, advice, resources, and much more. It is made up of different communities based on topics such as STEM, Common Core, Class Size, ELL, Funding, First Year Teaching, Special Education and much more. Each community has resources, a forum, and more.
It's a great, free resource to connect with other educators and share and learn.
Discovery Educator Network - great resources and great addition to any PLN
Creating a Personal Learning Network
Social Media in Education - connect, share, learn, communicate and more
Teachbook - Online Community for Teachers
Edudemic - online community with education and tech news