DWP Accessibility Manual
The DWP Accessibility Manual is one of the projects I'm most proud of. It is a collection of accessibility guidance and best practice all in one place.
I've contributed countless hours, pushed over 60,000 lines of code, written pages and pages of guidance, but I want it to be owned by the community. It's completely open source. View the DWP Accessibility Manual on GitHub, contrubute to it, and feel free to reuse any of the codebase for your own projects.
The project is based heavily on the GOV.UK Design System, only it is using a different configuration behind the scenes to make it feel more like the GOV.UK Prototype Kit. It uses Nunjucks and Markdown, so pages and guidance are easy to update and contribute to.