CoCalc is described as 'Web-based cloud computing (SaaS) and course management platform for computational mathematics' and is an website in the Development category. There are three alternatives to CoCalc, not only websites but also apps for Self-Hosted solutions, Windows, Linux and Mac. The best alternative is Overleaf, which is both free and Open Source. Other great sites and apps similar to CoCalc are Jupyterlab (Free, Open Source) and Colaboratory (Free).
Colaboratory is a free Jupyter notebook environment that requires no setup and runs entirely in the cloud. With Colaboratory you can write and execute code, save and share your analyses, and access powerful computing resources, all for free from your browser.