Alternative Operating System: MINIX
Alternative Operating System: MikeOS
A very limited and simple operating system written in assembler for 286 computers (and newer compatible architechtures, obviously). Pretty much what I would imagine as a successful outcome if I would write one to see that I can do it. It works, there's a text editor, a game, you can list, edit and execute files. Not much more though at first glance. I didn't look into writing additional software for it myself, yet. There are many forks of MikeOS. Most of them named after the forker and not under active development. It's a project I'd look into if I'd want to learn x86 assembler. Simple, not looking like any other OS I've seen.
Alternative Operating System: Haiku OS
Haiku OS is a BeOS clone. I didn't use BeOS back in the day (although I wish somebody would have showed it to me). So I'm not sure, but Haiku seems to be pretty much the same experience. But Haiku is open source, still actively developed and compatible with newer hardware. It ran relatively well on the Core2Duo PC I've tested it on. Except for the included web browser. That thing crashed. For a lot of people whether a desktop OS is usable is decided on how good of a web browser is available for it. Haiku OS Beta 3 looked promising with its WebPositive using WebKit 612.1.21. But at least on the old PC I've tested it on it wasn't usable. It was slower than imaginable and kept crashing after one or two page loads. (The simple included help pages at that. I didn't even feed it something complex, like YouTube or Google Docs.) But I've heard others hat a pretty good web experience with it. At least as long as nobody asks about security. The rest of the system is snappy enough. It's no KolibriOS, but on any x86 or x86_64 from the last ten years it should be as fast as anyone wishes their OS to be and much older computers run it just fine. There seems to be a not so small community of users and developers. Every new Beta that is released comes closer to a desktop OS that has everything that people ask about/for. (Let's not talk about big games people are familiar with.) And because of the growing community and the fact that the 32 bit version can still run many applications compiled for the original BeOS this is not just a small OS with theoretical goals bigger than its community. It's really usable already and it looks to me that it has good chances of becoming more important in the future. I'm not sure if I'd have said that five years ago. It's moving slowly (compared to Windows and Linux), but consistently towards its goals.
Alternative Operating System: Essence
Alternative Operating System: AspireOS