alt Hacker Newstake a look at AtheOS it's successor SyllableOS. created by a single developer, another single developer took it over (syllable) and it shortly became an open source project before it went defunct again. But it made impressive gains in the 3 years of initial development.
i miss those days of everyone and their mom creating an OS for giggles
Don't forget SkyOS. And there's plenty more, with SerenityOS being one of the latest notable examples. Those days never ended. Also ekhem ekhem TempleOS, as single developer as you can get.