There is an awesome article that goes over how one program can be written in different languages from the most abstract level of Math to Haskell to C to Assembly to actual machine code.
Tour de Babel - Funny de tour of programming languages.
Awesome static analysis - Static analysis tools for all programming languages, build tools, config files and more.
Twelf - Language used to specify, implement, and prove properties of deductive systems such as programming languages and logics.
GitHut - Place to discover languages on GitHub.
Semantic - Haskell library and command line tool for parsing, analyzing, and comparing source code.
Hello, World! - Hello world in every computer language.
TabNine - All-language ML autocompleter.
comby - Tool for changing code across many languages.
Quine - Self-referential program that can, without any external access, output its own source.
NEAL - Language-independent code analysis tool that aims to enable more people to write quality enforcement rules.
Bitsy - Best language to implement when writing your first compiler or interpreter.
codespan-reporting - Beautiful diagnostic reporting for text-based programming languages.
Hoist - Visual programming language inspired by Excel & Datalog.
Esoteric.Codes - Languages, platforms, and systems that break from the norms of computing.
Call-By-Push-Value - New programming language paradigm, based on the slogan “a value is, a computation does".
Zoekt - Fast text search engine, intended for use with source code.
PLaST research group - Focuses on applied programming languages and software engineering.
A Foundation for Typed Concatenative Languages (2017) - Presents a sound and complete type inference algorithm.
Polygott - Base Docker image for the Repl.it evaluation server.
teaberry - Learning project to implement a typed functional programming language/dialect of ML, which aims to be accessible and usable like Python is.
Langkit - Tool whose purpose is to make it easy to create syntactic and semantic analysis engines. Language creation framework.
remark-prism - Syntax highlighter for markdown code blocks using Prism.
PLT research - Collection of PLT researching.
ALGT - Abstract Language Generation Tool.
Language resources - Links on how to learn how to create programming languages.
CodeIntellify - Adds code intelligence to code views on the web.
go-enry - Faster file programming language detector.
Scriptometer - Measuring the ease of SOP (Script-Oriented Programming) of programming languages.
FeOFu - Compiler Seminar - Ideas that would lead to better compilers for existing languages, better future programming languages, better APIs and better performance software solutions in the future.
Quine Relay - Ruby program that generates Rust program that generates Scala program that generates.
GumTree - Complete framework to deal with source code as trees and compute differences between them.
Linguist - Library is used on GitHub.com to create languages used breakdowns.
Language Explorer - Explore 650+ programming languages, visualized based on paradigm influence relationships between languages.
FSL.tools - Programming language to make complex Finite State Machines easy to create and maintain.
[Introducing Dawn (Part 1) (2021)](https://www.dawn-lang.org/posts/introducing-dawn-(part-1)/) (Lobsters)
Langs In Rust - List of programming languages implemented in Rust, for inspiration.
treefmt - One CLI to format the code tree.
Sereal - Fast, compact, schema-less, binary serialization and deserialization oriented towards dynamic languages.
plyg - Playground to safely try out different programming languages in a single file.
Langium - Language engineering tool with built-in support for the Language Server Protocol.
Lizard - Code complexity analyser without caring about the C/C++ header files or Java imports, supports most of the popular languages.