Cool Collaborative PDF annotation tool : http://nb.mit.edu. Have your course PDF material on line, where students can enter notes, post questions and get answers!
Piazza is an online forum environment designed specifically for courses. It provides very rich control over registration (open or closed), student vs. instructor capabilities, web and email notifications, mobile apps, searching, tagging, response editing, and statistics. It takes only a few minutes for an instructor to set up a Piazza forum for a course.
For a tutorial see : http://www.slideshare.net/phsoffer/piazza-instructors-presentation