Check out Wiggio.
"Wiggio is the easiest and free way to work in groups."
Do your students find it hard to follow discussion threads and find the interface of your LMS unintuitive? Piazza is an online platform that provides a Q&A environment along with key features for effective course collaboration. The developer has tried to model the face-to-face discussions between students and instructors by displaying updates as soon as they happen. The platform also has tools like wiki-style editing and instructor feedback.Check out the web site to see real classes in action and the user friendly interface design. This is a great tool to foster online collaboration and discussions in your course.