Just what it says. The author had read the handbook soon after it came out and has revisited it 25 years later. He also offers interesting comments about most of the competing operating systems. See also
Making old systems accessible via the web https://athornton.dreamwidth.org/14733.html
Exploring old unixes https://athornton.dreamwidth.org/14340.html

MirageOS is a library operating system that constructs unikernels for secure, high-performance network applications across a variety of cloud computing and mobile platforms.
Mentioned as similar to IBM's VM on https://minnie.tuhs.org/pipermail/tuhs/2022-May/025771.html

We use rigorous formal methods to develop trustworthy software systems—systems that come with provable security, safety and reliability guarantees.

Maemo Leste continues the legacy of Maemo. We aim to provide a free and open source Maemo experience on mobile phones and tablets like the Nokia N900, Motorola Droid 4, Motorola Bionic, Pinephone, Pinetab, Allwinner tablets and more!

A nontechnical reminiscence written in 1988 (on the occasion of unplugging Columbia University's last DECSYSTEM-20) for a Digital Press book that was to commemorate DEC's 36-bit machines with a series of articles, but was never published.

Virtual CPU with full user/supervisor and virtual memory support, fast enough to support self-emulation.

Fast C-subset compiler allowing on-the-fly compile-and-go of all applications.

Network clients & servers, remote OpenGL, GUI applications, and many other nifty features.

Runs under Windows or Linux.

See the 00README.txt for full details and a step-by-step walkthrough tutorial.
Found via https://minnie.tuhs.org/pipermail/tuhs/2017-January/009247.html

This website provides information about the creation of operating systems and serves as a community for those people interested in OS development with 673 wiki articles.

This document is a compendium of Casper Ti. Vector's thoughts about the Unix philosophy in the contemporary context.

Instead of trying to create a single, static firmware, OpenWrt provides a fully writable filesystem with package management. This frees you from the application selection and configuration provided by the vendor and allows you to customize the device through the use of packages to suit any application. For developers, OpenWrt is the framework to build an application without having to build a complete firmware around it; for users this means the ability for full customization, to use the device in ways never envisioned.

An embedded operating system designed for running multiple concurrent, mutually distrustful applications on low-memory and low-power microcontrollers.