DJGPP is a complete 32-bit C/C++ development system for Intel 80386 (and higher) PCs running DOS. It includes ports of many GNU development utilities. The development tools require a 80386 or newer computer to run, as do the programs they produce. In most cases, the programs it produces can be sold commercially without license or royalties.
See also https://en.wikipedia.org/wiki/DJGPP
Found on https://cscope.sourceforge.net/index.html

Home page of Ali Gholami Rudi. Found on fefe's homepage
https://www.fefe.de/
His troff implementation Neatroff has been mentioned on TUHS https://minnie.tuhs.org/pipermail/tuhs/2022-January/025071.html
See also https://github.com/aligrudi/neatcc, a a small ARM/X86(_64) compiler for a subset of C; a brief introduction can be found on https://litcave.rudi.ir/neatcc.pdf, a small test suite on https://litcave.rudi.ir/ncctest.tar.gz

Zig is a general-purpose programming language and toolchain for maintaining robust, optimal, and reusable software.

Found in https://minnie.tuhs.org/pipermail/tuhs/2018-February/014546.html

The Amsterdam Compiler Kit is a cross-platform compiler and toolchain suite that is small, portable, extremely fast, and extremely flexible. It targets a number of low-end machines including the Z80, 8086 and 80386, but there are many other code generators available. It supports several languages, including ANSI C, Pascal and Modula-2, and contains integrated runtime libraries including a libc.

The maintainer, David Given states "I, dtrg, cannot honestly recommend using the ACK for production code unless as a stop-gap measure or unless the other benefits of the ACK (e.g. having a very lightweight turnkey toolchain is valuable to you) outweigh the code quality."

Main page is http://cowlark.com/cowgol/
It used to support the Apollo Guidance Computer used in the Apollo spacecraft, but I had to remove the code generator while rewriting the compiler and I haven’t reworked the AGC backend.
http://cowlark.com/2019-07-20-cowgol-agc/index.html
The author also ported the Fuzix operating system to the Raspberry π pico.
http://cowlark.com/2021-02-16-fuzix-pi-pico/index.html

His tool https://github.com/davidgiven/narcissus to remap keys might be interesting too.
So is his Forth system http://cowlark.com/fforth/

Loko Scheme, an optimizing R6RS Scheme compiler!

- Batteries on the side (Akku.scm)
- Only available for Linux/amd64, NetBSD/amd64 and bare metal amd64
- New, untested, obnoxiously Free Software
- Barely a REPL & slim documentation
- The worst of Scheme and Lisp

LFortran is a modern open-source (BSD licensed) interactive Fortran compiler built on top of LLVM. It can execute user’s code interactively to allow exploratory work (much like Python, MATLAB or Julia) as well as compile to binaries with the goal to run user’s code on modern architectures such as multi-core CPUs and GPUs.

See the output of several compilers in your web browser.
In addition to C++, it now supports Go, Rust and D.

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.