Found via a post on the TUHS mailing list
https://minnie.tuhs.org/pipermail/tuhs/2019-September/018465.html

GriCal (which is a short form for Grid Calendar) is an open web where people store quickly a few information about interesting events, so that others can also easily know about them. GriCal also makes easy and fast to get notified and store interesting events in its own external calendars.

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.

mpsh is an experimental command interpreter for Unix systems written by Dave Fischer at the Center for Computational Aesthetics and distributed under the 3-clause BSD license.

GNU Mes brings a Reduced Binary Seed bootstrap to GuixSD and potentially to any other interested GNU/Linux distribution, and aims to help create a full source bootstrap as part of the bootstrappable builds effort.

It consists of a mutual self-hosting Scheme interpreter written in ~5,000 LOC of simple C and a Nyacc-based C compiler written in Scheme. This mes.c is being simplified to be transpiled by M2-Planet.
https://github.com/oriansj/M2-Planet

The Scheme interpreter (mes.c) has a Garbage Collector, a library of loadable Scheme modules– notably Dominique Boucher's LALR, Pre-R6RS portable syntax-case with R7RS ellipsis, Matt Wette's Nyacc –and test suite just barely enough to support a simple REPL and simple C-compiler: MesCC.
https://github.com/oriansj/mescc-tools

Mes+MesCC can compile an only lightly patched TinyCC that is self-hosting. Using this tcc and the Mes C library we now have a Reduced Binary Seed bootstrap for the gnutools triplet: glibc-2.2.5, binutils-2.20.1, gcc-2.95.3. This is enough to bootstrap GuixSD for i686-linux and x8664-linux.

Mes is inspired by The Maxwell Equations of Software: LISP-1.5 – John McCarthy page 13, GNU Guix's source/binary packaging transparency and Jeremiah Orians's stage0 ~500 byte self-hosting hex assembler.

GNU Mes is free software, it is distributed under the terms of the GNU General Public Licence version 3 or later. See the file COPYING.

cl-6502 is a Common Lisp emulator, assembler and disassembler for the MOS 6502 processor.

Found via computable-reals https://github.com/tarballs-are-good/computable-reals, but others such as Quickproject https://github.com/tarballs-are-good/quickproject are interesting as well.

NeoPG is a modern replacement for GnuPG 2
NeoPG starts as an opinonated fork of GnuPG 2 to clean up the code and make it easier to develop.
We want to provide a stable and extensible API for application developers, too.
Eventually, we will add new ways to use OpenPGP that make it accessible and usable.

Small as a mouse, fast as a cheetah and available for free. NetSurf is a multi-platform web browser for RISC OS, UNIX-like platforms (including Linux), Mac OS X, and more.