Minix

Minix is the Unix-like operating system that inspired Linux in the first place. It's a very tiny microkernel based OS, written primarily to teach the design of a Posix-compatible operating system. Since then, it's had some rewrites, and is now capable of operating your home computer. They would like some additional driver support, and offer their OS with a BSD license, which allows you to do...pretty much everything except claim you wrote it or sue.
When I tried it out, I was expecting a repeat of my horrible experience with HURD, but Minix was different. Yeah, they're both microkernel Unix-alikes, but Minix was far less fragile. Everything ran. Everything ran reasonably fast. There was extra software, and it installed without the usual whining and carrying on from the OS, and nothing broke when I installed something else. Everything just flat out worked. Part of the reason for this may be the "reincarnation server" that keeps copies of critically important programs, and can kick-start them again if something makes them stop working, whereas in other microkernel operating systems, a failure in anything critical means you're restarting. It was a Linux-like experience....but slightly faster, because all the code was smaller. Sweet.
On the downside, Minix insists on its own weird little partitioning scheme, which confused me. I also dislike the bootloader, in which a mini-os loads before the main OS and requires your intervention, whereas other unix-likes can be left alone until login time. That was kind of annoying. I also worry that if I screwed something up, that I'd be permanently locked out of the system. In Linux systems, I do have ways to break back in and take back my files if the OS screws up. Or, more likely in Minix's case, I forget my passwords.
I feel like it would make a great install-to-ram bootstrapping OS, but I can't find any documentation on how to do that. Oh well. My experiments on this were some time ago, so I've forgotten a lot about it since then.