Found on Sandra Snan's notice https://idiomdrottning.org/notice/AgXNxNMyxT1ZMgpLPM

The PX5 RTOS is ULTRASMALL (< 1KB for minimal use), enabling its use in some of the most memory-constrained devices. It is one of the smallest RTOSes available, requiring less than 1KB of FLASH and 1KB of RAM on typical 32-bit microcontrollers. I have not found out how it is licensed.
The company also offers https://px5rtos.com/px5-net/, a TCP/IP implementation.

Found on https://www.theregister.com/2023/11/28/microsoft_opens_sources_threadx/

Enchive is a tool to encrypt files to yourself for long-term archival. It's a focused, simple alternative to more complex solutions such as GnuPG or encrypted filesystems. Enchive has no external dependencies and is trivial to build for local use. Portability is emphasized over performance.

Supported platforms: Linux, BSD, macOS, Windows

The name is a portmanteau of "encrypt" and "archive," pronounced en'kīv.

Files are secured with ChaCha20, Curve25519, and HMAC-SHA256.

Key pairs can be derived from a pass phrase.  See https://github.com/skeeto/passphrase2pgp for a tool to generate gpg key pairs in a similar way.

Document explaining the development of a bc/dc clone. Might be a good example to follow, if I ever make a public project.

Software that aims to provide an easy to use (hopefully) universal blackbox for solving polynomials and secular equations.

Among its features you can find:
- Arbitrary precision approximation.
- Guaranteed inclusion radii for the results.
- Exploiting of polynomial structures: it can take advantage of sparsity as well as coefficients in a particular domain (i.e. integers or rationals).

It can be specialized for specific classes of polynomials. As an example, see the roots of the Mandelbrot polynomial of degree 2.097.151 computed in about 10 days on a dual Xeon server.

If you use MPSolve in your research, please cite it as follows:
Bini, Dario A., Fiorentino, Giuseppe, Design, analysis, and implementation of a multiprecision polynomial rootfinder. Numerical Algorithms 23.2-3 (2000): 127-173.
Bini, Dario A., and Robol, Leonardo. Solving secular and polynomial equations: A multiprecision algorithm. Journal of Computational and Applied Mathematics 272 (2014): 276-292.
Found via https://en.m.wikipedia.org/wiki/Aberth_method and https://en.m.wikipedia.org/wiki/MPSolve
See also the GitHub repository http://github.com/robol/MPSolve.git A simpler implementation of the Ehrich-Aberth-method can be found in https://github.com/robol/ea-roots

primecount is a command-line program and C/C++ library that counts the number of primes ≤ x (maximum 1031) using highly optimized implementations of the combinatorial prime counting algorithms.
Found via https://www.cliki.net/primecount and https://github.com/AaronChen0/primecount

Implementations in Basic and C.

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

See also https://github.com/vsedach/Vacietis and https://awesomeopensource.com/project/vsedach/Vacietis

Small. Sloccount counts under 2000 lines of code, small enough to allow audits. The binaries can be under 50KB, small enough for many embedded targets.
Easy to deploy. Just add monocypher.c and monocypher.h to your project. They compile as C99 or C++ and are dedicated to the public domain (CC0-1.0, alternatively 2-clause BSD).
Portable. There are no dependencies, not even on libc.
Honest. The API is small, consistent, and cannot fail on correct input.
Direct. The abstractions are minimal. A developer with experience in applied cryptography can be productive in minutes.
Fast. The primitives are fast to begin with, and performance wasn't needlessly sacrificed. Monocypher holds up pretty well against Libsodium, despite being closer in size to TweetNaCl. (More detailed benchmark)