Ask HN: Where can I get a free domain to use with GitHub Pages?

I’m building an open-source project and plan to host its documentation on GitHub Pages. I’d like to get a free (or at least very cheap) custom domain to make it look more professional. Ideally, the domain should be easy to obtain, require minimal setup, and work well with GitHub Pages’ HTTPS support.

What services or registrars do you recommend for this? Are there any pitfalls or limitations I should be aware of when using a free domain for a public project?