[Tomte]

I've never tried HaikuOS, but it's high on my list of things to look at. Sometime. It's BeOS revitalized.

Also Minix3. An academic microkernel (although it runs NetBSD userland now, so maybe a bit practical).

And if I could ever get my hands on it, QNX. A practical microkernel.

MenuetOS is a pure assembler based OS that seems nice.

[prima-facie]

I've tried most of them. I was impressed with BeOS back in the day and have used it for a few weeks as the main OS on my machine. I tried Haiku for a short time a few years ago. It had potential to be a great OS but it has become irrelevant now. The industry has moved on at such a fast pace that Haiku would need many man-years of development to catch up. I look with sympathy at the development team and their effort knowing that one day they'll lay down their tools and stop working on it for good.

I've only tried QNX very briefly about 15 years ago or so. It had the Neutrino kernel as far as I remember. Unfortunately I don't have any deep insight into it. It's basically a RTOS, with a micro-kernel, with a UNIX compatible environment. I remember the company advertising that it can work on industrial-grade equipment, where a RTOS is needed. I'm surprised to see that Google is now developing something very similar with Magenta but aimed at the desktop.