I started SageMath in 2004 to provide a FOSS alternative to expensive commercial mathematics software. Sage is Python-based and has had around 600 volunteer contributors. The project has also received millions of dollars in support from grants around the world, and has a very active developer community.
This site is about Software as a Service, and there are at least two easy ways to use Sage online as a service:
Do you know an article comparing Sage Math to other products?
You can submit it here