The community is huge and generous enough that it has open-sourced tons of complex projects.
First off, I would like to thank all the open-source contributors who are making the world a better place. I hope you would be able to use some of the following resources and return the favor by contributing to our open-source community. Cheers to these unsung heroes!
As a curious software engineer, I'm always looking for the open-sourced alternatives of popular solutions. This post covers opensource alternatives of Notion, Google docs & Grammarly.
Also, don't expect that you will get a complete solution for free. These opensource projects may not have the exact functionality and tons of features but they do cover the basic idea of the product.