PRs

• check_run: individual task of an external CI
• check_suite: suite of tasks of an external CI
• pull_request: Triggered on actions around PRs
• pull_request_review: review of a PR
• pull_request_review_comment: comment on PR in the code

GitHub Projects (Kanban boards)

• project: general actions around a project
• project_card: actions related to tasks with a card on a project
• project_column: actions related to editing a column in a project

Releasing / Deployment

• deployment: when a deployment is created or modified
• deployment_status: when a deployment's status changes
• page_build: pushed to a GitHub Pages branch
• release: when a release is made or modified

Org related

• member: when a person is added / removed as a collaborator on a repo

Issues

• issues: general actions around an issue
• issue_comment: comment on an issue

General Git

• create: create a tag / branch
• delete: delete a tag / branch
• push: make a push of any kind (commit or tag)
• commit_comment: someone makes a comment on a particular commit

Misc

• schedule: trigger based on cronjob schedule
• repository_dispatch: Webhook for 3rd party access
• public: when a private repo goes public
• watch: when someone stars a repo
• fork: when a repo is forked
• gollum: when pages in the repo's wiki are edited
• label: actions around label modification
• milestone: when GitHub Milestones are edited
• status: when the CI status of a commit changes