Ask HN: Tools/services normally paid for but free for open source products?

Hi HN, What are some cool tools that are normally paid but offer something free for open source projects?

Examples I have collected so far:

- Auth0 (https://auth0.com/pricing)

- GitLab (https://about.gitlab.com/2018/06/05/gitlab-ultimate-and-gold-free-for-education-and-open-source/)

- GitHub (possibly not current)

- 1password (https://github.com/1Password/1password-teams-open-source)

What are other interesting / good tools that are free for open source projects?