CoCalc
There is
an online service
for running SageMath computations online.
It has been redesigned from the ground up to be much more reliable,
sustain higher load and offer a lot more features.
It can be considered as the successor of the no longer existing "SageNB" online service.
Start
Chrome Extension
Existing Files
To transfer existing *.sws files to the new CoCalc service,
download the notebook files from your current SageNB setup and upload them into a new project on CoCalc.
When you open them, they'll be converted to SageWorksheets.