mentioned in this thread:
C
* Lua https://www.lua.org/source/
* Redis https://github.com/redis/redis (mentioned twice)
* Ruby https://github.com/ruby/ruby
* SQLite https://sqlite.org/src/dir?ci=trunk
C++
* Botan https://github.com/randombit/botan
* ClickHouse https://github.com/ClickHouse/ClickHouse
Go
* Go's standard library https://cs.opensource.google/go/go
* HashiCorp repos, particular those by Mitchell https://github.com/hashicorp
F#
* jet.com repos https://github.com/jet?language=f%23
Haskell
* Elm compiler https://github.com/elm/compiler
Lisp
* CL-PPCRE https://github.com/edicl/cl-ppcre/
* Mezzano USB driver https://github.com/froggey/Mezzano/blob/master/drivers/usb/m...
PHP
* Carbon https://github.com/briannesbitt/Carbon
* Laravel https://github.com/laravel/laravel
Python
* pytudes (notebooks) https://github.com/norvig/pytudes
R
* sf (ex of functional OOP) https://github.com/r-spatial/sf
Ruby
* Sequel https://github.com/jeremyevans/sequel/
* Sidekiq https://github.com/sidekiq/sidekiq
Rust
* ripgrep https://github.com/BurntSushi/ripgrep
Scala
* fs2 (ex of good type safety) https://github.com/typelevel/fs2
* monix (ex of ugly code with great performance) https://github.com/monix/monix
TypeScript
* GitHub Desktop (ex of SPA) https://github.com/desktop/desktop
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thanks to: cejast, bb86754, wewxjfq, antonyt, bit, diego_moita, agjmills, freedomben, Contortion, valenterry, DethNinja, mberning, seneca