GitHub uses GitHub Flavored Markdown
Documentation, including automatically rendered README files in a variety of Markdown-like file formats (see README § On GitHub)
Wikis
GitHub Actions, which allows building continuous integration and continuous deployment pipelines for testing, releasing and deploying software without the use of third-party websites/platforms
Graphs: pulse, contributors, commits, code frequency, punch card, network, members
Integrations Directory
Email notifications
Discussions
Option to subscribe someone to notifications by @ mentioning them.
Emojis
Nested task-lists within files
Visualization of geospatial data
3D render files can be previewed using a new integrated STL file viewer that displays the files on a "3D canvas." The viewer is powered by WebGL and Three.js.
Support for previewing many common image formats, including Photoshop's PSD files
PDF document viewer
Security Alerts of known Common Vulnerabilities and Exposures in different packagesGitHub's Terms of Service do not require public software projects hosted on GitHub to meet the Open Source Definition. The terms of service state, "By setting your repositories to be viewed publicly, you agree to allow others to view and fork your repositories."