BeOS/Haiku
AmigaOS/AROS
Plan 9
FreeDOS
Minix
I too am intrigued by KolibriOS. Assembler coded; man that must scream on a modern processor. But I have no machines with working floppies. Will it boot off a 5 in 1 card reader (SD)? Or USB thumb drive?
They left out FreeVMS, which of the alternative Open Source OSes I've heard of, I'd be mostly likely to give a try if I were really, really bored.
KolibriOS just sounds insane. Writing an OS in assembly language for a class project is rather fun (mine was in HP1000 assembly), but has rather limited application. Kind of like something that is fun to say you did, but know you will never do again.
When it comes down to it, I'm a Unix guy, heart, body and soul. I've run Unix at home for over a quarter century. It has aged extraordinarily well. I'm not looking for a replacement.
Microsoft Windows(tm) is eventually going to wither on the vine. Closed systems are like so mid 20th century. I would expect that there will not be another major system done beyond Unix (=Linux/BSD/Darwin) and something Windows (=Microsoft/ReactOS). The cleaner design of Unix means it will continue to age well. The clear separation of layers means you can pretty much do anything you want to with it. I recommend the Vista haters take a hard look at ReactOS. That is your future.
(I'm rooting for the ReactOS guys to beat Microsoft. Once that happens, the idiots who program computer games will have no choice but to make their games run on Open Source systems. Of course, profitable game companies like Blizzard already understand that).