SageMathCell project is an easy-to-use web interface to a free open-source mathematics software system SageMath.
It allows embedding Sage computations into any webpage: check out short instructions or comprehensive description of capabilities.
You can easily set up your own server.
There are a lot of resources available to help you use Sage. In particular, you may ask questions on sage-support discussion group or ask.sagemath.org website.
If you experience any problems or have suggestions on improving this service (e.g., you want a package installed), please email Andrey Novoseltsev.
SageMathCell is expected to work with any modern browser and without any downtime.
Need more power and flexibility but still prefer to avoid your own installation of Sage? CoCalc will allow you to work with multiple persistent worksheets in Sage, IPython, LaTeX, and much, much more!