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."

For a long time, we were unhappy with having to care about security issues and Linux distribution maintenance on our various Raspberry Pis.
Then, we had a crazy idea: what if we got rid of memory-unsafe languages and all software we don’t strictly need?
Turns out this is feasible. gokrazy is the result.

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/

Detailed description for using a network root filesystem on a Raspberry π. Has lots of details, for example I learned about the fatlabel program.

Just what it says. A 4 GB Rπ costs £8.63/month ≈ €9.57/month.

Highly optimised minimal Debian OS

DietPi is extremely lightweight at its core, our images start at 400MB in size (3x lighter than 'Raspbian Lite'). With features of low process/memory footprint and DietPi-RAMlog installed by default, DietPi allows you to get the maximum performance from your device.

Brief description of a failed attempt to install the virtual AGC
from https://www.ibiblio.org/apollo/ on a Raspberry π 4 running Raspian.

Probably using the Spigot Algorithm from Stanley Rabinowitz.

How to install FreeBSD/ARM on computers of the Raspberry Pi family.