Groups

Anything related to the karger thompson attack: proof of concept demos, mitigations, theory.

multics the original paper explaining the attack (before thompson!)
SCM Security by Wheeler (Secure distribution & compilation of source fundamentals; Karger advised mastering it)
rotten by rntz (thompson attack demo)
rust infection by manishearth (thompson attack demo in the rust compiler)
tcc ACSAC by daved wheeler
CompCert by Leroy et al (Mathematically-verified, C compiler whose specs and proofs checked with tiny, verified checker)
CakeML by Myreen et al (Mathematically-verified, SML compiler whose specs and proofs checked with different, tiny, verified checker)
VLISP by Oliva and Wand (Article has links to VLISP which mathematically verified PreScheme and Scheme48)
KCC by Rosu et al (Executable, formal semantics for C in rewrite logic; could do that w/ simpler engine)
TALC by Cornell (Typed, assembly language to verify safety w/out compiler; checker can be simple; C subset + verified compiler to TALC)
CoqASM by Microsoft Research (Bootstrap in verifiably-safe assembly in prover checked by tiny, verified checker)

These are tools written in ubiquitous languages, therefore they can be used in a wide variety of contexts.

shasm by Hohensee (x86 assembler written in BASH)
AWKLisp by Bacon (LISP written in Awk; includes Perl version from Perl Avenger)
Gherkin by Dipert (LISP written in Bash)
BASH Infinity by Brzoska (BASH framework/routines that might help write compilers in it)
mal "make a lisp" implementing a very basic lisp interpreter in hundreds of languages
[1] A new bootstrapping project that is built up to a self host language above assembly from a minimal DOS platform.

c4 by rswier (incredibly short c compiler)
cc500 by edmund grimley-evans (tiny c compiler)
CUCU by Zaitsev (Small, C compiler designed for easy understanding)
SmallerC by Frunze (Small, single-pass, C compiler for several ISA's)
picoc interpreter.
C Interpreter by Dr Dobbs (Describes building a C interpreter with source)
[2] Small C for I386 (IA-32)
Selfie, a tiny self-compiling compiler for a subset of C, a tiny self-executing MIPS emulator, and a tiny self-hosting MIPS hypervisor, all in a single 7kLoC file. HN discussion. Paper.
Tiny C expression compiler Written in Forth based on tinyc.c by marc feeley.
[3] [4] C compilers by Rui Ueyama blog
[5] 10 hour self hosting c compiler

Grammar Executing Machine by McKeeman and He (Incrementally extend languages from simple to complex grammars in interpreter(s))
peg by kragen (parsing)
PEG-based simple compiler by Ian Piumarta
META II by Bayfront Tech (Original meta-compiler w/ live code and detailed tutorial; OMeta was successor)
META II implementation by Lugon (Looks like a small implementation of META II; also bootstrapped in META II)
OMeta# Intro by Moser (OMeta intro that nicely illustrates the meta approach/advantages)

P-code by Wirth (High-level language & libraries target ultra-simple, portable interpreter)
sweet16 by Steve Wozniak
Tiny BASIC by Allison (Small BASIC whose original VM took 120, virtual opcodes to implement using 3KB RAM)
Klip by Cutting (Compiler & runtime for simple language for students; done in C#; runtime is very readable)
Uxn by Linvega (Portable VM with 32 mnemonics implemented in 100 lines of C89; self-hosted assembler)

NAND2Tetris by Nisan and Schocken (Guide that teaches hardware step-by-step in fun way with simple CPU emerging)
J1 by by Bowman (16-bit Forth CPU in 200 lines of Verilog that does 100MIPS on FPGA's)
H2 by Howe (Modified, VHDL version of J1 with detailed description and Howe's code MIT-licensed)
RISC-0 by Wirth (Simple, RISC CPU & SOC designed for Oberon language with detailed docs and source online)
JOP by Shoeberl et al (Embedded Java processor that takes up 1830 slices on FPGA)
Scheme Machine by Burger (Scheme interpreter implemented as CPU using formal methods)
ZPU by Zylin AS (Tiny, 32-bit CPU for deep embedded apps in 440 LUT's)
J2 by Landley et al (Clone of cost-efficient, SuperH-2 CPU in open-source)
VAMP by Beyer et al (Formally-verified, DLX-style processor in 18,000 slices on Xilinx)
Leon3 by Gaisler (Industry-grade, 32-bit SPARC w/ auto-configuration of core and GPL license)
Rocket by Univ of CA (1.4GHz RISC-V CPU and generator for customization)
OpenPITON by Princeton (25-core, shared-memory, SPARC CPU open-sourced and very scalable)

KolibriOS - lightweight assembly OS.
MikeOS - same.
Sortix - modern reimplementation of POSIX in C. (Note: No perl port and GCC does not build natively on it. (yet.))
ASMLINUX - linux kernel, but the userspace is implemented entirely in assembly.
LFS - Guide on building Linux and the GNU userspace.
buildroot
NetBSD build.sh - Cross-build a complete NetBSD ISO from a foreign OS. There's also a guide in the official NetBSD docs.
lh-bootstrap - alternative linux distro, using musl instead of glibc.
xv6 - UNIX teaching OS MIT
OS/161 - UNIX teaching OS Harvard
https://landley.net/toybox/about.html - Toybox, alternative to Busybox by Robert Landley, see also Aboriginal Linux and mkroot by the same author, which are all geared toward a minimal boostrappable system
https://github.com/pikhq/bootstrap-linux - Another take at a bootstrappable Linux system
Project Oberon - Operating system and programming language Oberon (from the creator of Pascal - Niklaus Wirth)
TinyGo Operating system in programming language Go
TamaGo - Another operating system in programming language Go
seL4 Microkernel - OS kernel in less than 10.000 lines of source code and less than 150 KB binary
Ultibo - Unikernel for Raspberry Pi written in FreePascal