HelenOS is a portable microkernel-based multiserver operating system designed and implemented from scratch. It decomposes key operating system functionality such as file systems, networking, device drivers and graphical user interface into a collection of fine-grained user space components that interact with each other via message passing. A failure or crash of one component does not directly harm others. HelenOS is therefore flexible, modular, extensible, fault tolerant and easy to understand.

Rump kernels enable you to build the software stack you need without forcing you to reinvent the wheels. The key observation is that a software stack needs driver-like components which are conventionally tightly-knit into operating systems — even if you do not desire the limitations and infrastructure overhead of a given OS, you do need drivers.
Found on Kooda's blog https://www.upyum.com/en/post/2.xhtml

NuttX is a real-time operating system (RTOS) with an emphasis on standards compliance and small footprint. Scalable from 8-bit to 32-bit microcontroller environments, the primary governing standards in NuttX are Posix and ANSI standards. Additional standard APIs from Unix and other common RTOS's (such as VxWorks) are adopted for functionality not available under these standards, or for functionality that is not appropriate for deeply-embedded environments (such as fork()).

Emacs-like editor running in the web browser.

Capsicum is a lightweight OS capability and sandbox framework developed at the University of Cambridge Computer Laboratory, supported by grants from Google, the the FreeBSD Foundation, and DARPA. Capsicum extends the POSIX API, providing several new OS primitives to support object-capability security on UNIX-like operating systems:

   capabilities - refined file descriptors with fine-grained rights
   capability mode - process sandboxes that deny access to global namespaces
   process descriptors - capability-centric process ID replacement
   anonymous shared memory objects - an extension to the POSIX shared memory API to support anonymous swap objects associated with file descriptors (capabilities)
   rtld-elf-cap - modified ELF run-time linker to construct sandboxed applications
   libcapsicum - library to create and use capabilities and sandboxed components
   libuserangel - library allowing sandboxed applications or components to interact with user angels, such as Power Boxes.
   chromium-capsicum - a version of Google's Chromium web browser that uses capability mode and capabilities to provide effective sandboxing of high-risk web page rendering.

i3 is a tiling window manager, completely written from scratch. The target platforms are GNU/Linux and BSD operating systems, our code is Free and Open Source Software (FOSS) under the BSD license. i3 is primarily targeted at advanced users and developers.

HelenOS is an open-source microkernel-based multiserver operating system written from scratch. It runs on several different CPU architectures (IA-32, x86-64, SPARC V9, IA-64, PowerPC, ARM, MIPS). HelenOS prides in portability, modularity, clean design and coding style.

It does not mimic any existing system and it is not meant to be POSIX-compliant or UNIX-like at all. Just as an example, HelenOS does not support fork().

wdfs is a webdav filesystem that makes it possible to mount a webdav share under linux, freebsd and apple mac os x. wdfs has some special features for accessing subversion repositories via webdav. it's based on fuse and neon.

How to use ZFS snapshots as backup mechanism.

The new home of Lispbox, a "ready to run" package of a Lisp system, Emacs, SLIME and some libraries. Work in progress.