Igalia's post on self-hosting CI runners is a masterclass in build engineering. It shows dealing with weird API constraints, but still finding good designs.
I also learnt that some CI systems use OCR on Apple's installers to automate!
I really admire projects that put their goal in their name. For example, lastpass.
It's a great way of staying focused.
I once built a project called "one click deploys" to automate a manual deployment. The name stopped us adding unnecessary clickthroughts on big systems.
Find yourself regularly reviewing pull requests just to comment "please update the changelog"? Automate it! https://danger.systems/js/
Unlike a CI tool, this is automating PR commenting.