(I'm the founder of SageMath.) SageMathCloud (https://cloud.sagemath.com) is a highly collaborative online web application for working with Sage, R, Python, IPython, LaTeX, etc that I've been working extremely hard on this for almost 3 years. We'll have a major new release this week with much new polish and functionality (the testing server is at https://web0.sagemath.com). Everything is 100% open source. https://github.com/sagemathinc/smc