Edmodo is free and offers you a simple virtual learning environment where your students can share ideas and collaborate together. It has a very similar interface to Facebook and so your students will pick it up easily. What's more, students to do need provide their emails. It is really easy to create a group and then invite your students to join the group. You can create separate groups for each class you teach. What is more amazing is that it is free. You can create quizzes, polls, add up important dates to the calendar, run discussions and share links, videos, articles etc.