MenuetOS
This is a really interesting one. Or two. I'll start with MinuetOS. Written in Assembler for 586 systems, open source, very impressive, performance- and size-wise. Including the included applications it fits in 1.44 MB. The boot time on my Core2Duo isn't easily measured because it's completely done before my monitor adjusts the resolution from the boot menu to the desktop. But when I chose the same resolution it uses for its boot option screen, I learned that it boots up completely pretty much instant even if the monitor doesn't have to switch modes. A guess writing 100 % in Assembler makes this possible. Very promising software, but a small community. The developers have been very dedicated when the 32 bit version was still under active development. So it is very stable, packed with useful little applications, especially for development and debugging, but also for daily use and casual gaming. This 32 bit version is not actively developed anymore.
There is a 64 bit version, which is not open source. I'll mention the reason in the paragraph after the next paragraph. It's where the development of MenuetOS is happening nowadays. I don't know how far it has changed since the 32 bit version. I didn't try it, because I'm looking for a 32 or 16 bit system. So, the source is not available, but the information available should be suffice to write drivers and applications for it. Edit: I've tried the 64 bit version briefly. Since there has been years of relatively active development between the last (pre 1.0) 32 bit version and the latest 64 bit version, the changes and advantages are very noticable. If a closed source OS is an option, the 64 bit version is surely what you'll prefer using.
There is a CD image of ~ 22 MB of additional applications that I didn't really try. But it's worth mentioning that there are more than the very basic applications available. There's a media player, even some networking tools (FTP, telnet, but no SSH, as you might have guessed). There is a very rudimentary shell. But it is obvious that the developers' focus was on GUI programmes, as even the ping tool is used by opening a window, entering the target in a text field and starting the ping with a button. Most things worked flawlessly during the time I've tried MenuetOS. But there are a couple of things that may be considered annoying by a demanding user. The most obvious to me was that the mouse is very hard to use. Stopping the pointer in a particular spot takes practice. It seems to drag behind quite a bit. It takes time to do something with a mouse. And when setting the speed slower the pointer sometimes starts moving into the opposite direction. Not completely a matured OS. But many parts are matured and it is usable.
KolibriOS
And then there is KolibriOS, a fork of the 32 bit MenuetOS. It doesn't claim to be a fork, though. Its heritage isn't mentioned on the web site. Just once in the web forum by somebody asking about it. Every mention of it in the source code has been removed, but copyright notices have been added by the "new owner". That makes this fork a pretty vile act. I don't know what the motivations behind this have been. It is pretty clear that the KolibriOS developer didn't start KolibriOS from scratch, no matter how much they try to make it look like it. There are people who learn about and get into KolibriOS who don't know where it came from. So, after this fork the developer of MenuetOS has decided to not publish the source code of the new 64 bit version of MenuetOS. I felt it relevant to get into all that because making a decision for KolibriOS and against MenuetOS can be seen as a political statement. (And I'm not saying this in any way as a reference to current international political happenings. KolibriOS is sometimes called "a Russian OS". But people who speak out against using KolibriOS because it's Russian and therefore evil should either produce some sort of evidence, hint or at least rationale for why the OS or its developers have any connection to the war in Ukraine or shut up.)
So, back to the OS itself. It's not fully compatible with MenuetOS. As with previous versions of MenuetOS, some programmes still run, some have problems and many just don't work any more on the new version. I don't know if there are even some programmes left that have run on both systems.
There are still 1.44 MB images for floppy disks being published. But for my first impressions I chose the large image that comes with many more applications. That image is over 130 MB, which is likely mainly due to the applications that are not written in assembler. But still, a multiple of the CD image with additional software for MenuetOS. I'd have to compare them in detail to be able to say why that is. The set of applications that is included in the large KolibriOS image is very good for basic requirements and significanty surpasses what's usually included in OS images. There are editors, file browsers, development tools, quite a few casual games and a few not so basic applications. Some not written in Assembler just for this OS, like DOSBox and Netsurf, others impressively small and fast. Netsurf being the only web browser that's available for the OS makes clear who might want to consider using this OS and who might not.
The whole experience of using KolibriOS was a bit smoother than using MenuetOS. That is, almost everything was perfectly smooth, nothing ever crashed. But, some of the not so basic applications are more resource hungry than what you would expect from an OS written in assembler (which is because those applications aren't written in assembler and probably ported from another platform with functionality as the only requirement). For example, the video player would use hundreds of MB of RAM for the playback of some video files. The whole system would become less and less responsive, even though there were still GBs of RAM unused. I don't know what codex' are implemented and how. But some formats wouldn't play at all, bringing the entire system to a halt until I was able to kill the media player.
The size of the complete KolibriOS image (way over 100 MB, as compared to under 25 MB for MenuetOS) shows how the goal of keeping everything small and fast has been overlooked more and more when porting software to run on KolibriOS. But also that more application have been ported. Those are not part of the OS if you use the normal floppy or CD image. But there is one with everything pre-installed. The fact that those extra applications don't run as fast and aren't as small shouldn't be a problem and isn't a fault of the OS. But the fact that no alternatives have been written specifically for Kolibri might be restricting its use in practice.
The following screen shots are all of KolibriOS.