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 - Curated list of static analysis tools, linters and code quality checkers for various programming languages.
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.
Programming Languages Zoo - Collection of implementations of miniature programming languages.
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.