While you may never have heard of AmigaOS, it's been around for a couple of decades. Even the original version was a full preemptive multi-tasking OS with quite a nice GUI. According to this Ars Technica article, "Steve Jobs was reported to be extremely worried about the Amiga, but fortunately for him and Apple, Commodore had absolutely no idea what they were doing."
Back in 1985, it ran on the Commodore Amiga. Now, it runs on Power PC based systems. It's not clear from the site whether there's anyone OEM-ing hardware, but I assume there must be. Maybe it's directly installable on PPC Macs.
I won't be throwing my Macs out anytime soon, but it's nice to see a classic like this getting a new lease of life.