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 also run it on 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.