As a foreign language teacher, I find it incredibly important to have collaboration between students. Sometimes with me, sometimes together. Regardless of the topic, I want them to be able to work together.
I found Thinkbinder, a free tool where kids can "meet" virtually and review materials, collaborate, do projects, etc. as a group. It is an easy to use tool as well.
Start off by creating an account and generating a group. Once you are in, you invite members to the group and they login and join. It is a little like Edmodo in the aspect of group codes and moderation by a teacher. Once the kids are in, they can add files, collaborate, work together. It could be used for many different things: project planning, review for tests, small group meetings to do a task, the list goes on and on.
Check out this site. It's free and easy.