IntenseDebate - IntenseDebate's comment system enhances and encourages conversation on your blog or website.
Padlet - Visual boards for organizing anything.
DISQUS - Disqus is a global comment system that improves discussion on websites and connects conversations across the web.
BookWidgets - BookWidgets is an excellent application that let you create engaging exercises in minutes and easily share a varied variety of beautiful and fun activities with your classroom and others.
GraphComment - Add modern discussion and social interactions to your website
Quizlet - Quizlet allows you to review and create flashcards for a variety of subjects, such as math and reading.