300,000 members and over 265,000 resources, many of which are Common Core Aligned. CC badge given to resources which have been vetted by a panel of teachers at the AFT.
Apart from the vast quantity of resources, all of which are free, they also have an online Common Core information center with blogs, the history of CCSS, FAQs and links to resources based on subject.
Wikisapces: "Which is why we created our brand-new YouTube channel for all our video tours and instructional videos. Not only because it gives us a handy place to share tips and instructions that might be too confusing to describe in just words, but because it lets you, our users, share these videos with your members."
Dropbox is a very useful tool to store online files, sync your computers and mobile devices and share files. With all those capabilities it makes it a natural tool for educators!