Piazza is a free online gathering place where students can ask, answer, and explore 24/7, under the guidance of their instructors. Students as well as instructors can answer questions, fueling a healthy, collaborative discussion.
K. Cortis, S. Scerri, I. Rivera, and S. Handschuh. Social Informatics, volume 8238 of Lecture Notes in Computer Science, Springer International Publishing, (2013)