I tried Haiku recently on a not-super-old PC and was impressed by the speed and the reasonable availability of software. I then wished I could run this on my old Powerbook G4 because nothing else seems to run there anymore with ease, and this seems like the perfect desktop OS for such old machines. And it turns out there is now a fork for PPC: https://github.com/ActionRetro/Tabby-PPC !

(Why a fork? Because it's primarily vibecoded and Haiku won't take the patches upstream. But hey, I'm gonna give it a try anyway.)