Curriki

Curriki is an online community for creating and sharing best teaching practices and curriculum. The community contains lessons, units, assessments, and multimedia resources across all subject areas. Grade levels include K-12 and all material is free with sign-up. Teachers can use Curriki to build on their own curriculum. Outside of the classroom, Curriki is extremely valuable to those who may not have access to high-quality education. The most beneficial component of Curriki, the leading organization behind the OER movement, is that they are currently working to develop multilingual educational content. I didn't find any aspect of the community that is negative. 

Curriki is the leader behind the OER movement