A build daemon in Guile
When using Guix, you might be aware of the daemon. It runs in the background but it's a key component in Guix. Whenever you've been using Guix to operate on the store, whether that's building something or downloading some substitutes, it's the daemon managing that operation.
The daemon also is a key part of the history of Guix. The Guix project started mixing Guile with ideas from the Nix project, and the guix-daemon is a fork of the nix-daemon with some tweaks made over the years. Rather than being implemented in Guile though, the daemon is implemented in C++ with some helpers written in Guile. Given the focus on Guile in Guix, this is unusual, and I believe it's made working on the daemon less desirable, especially since rewriting it in Guile has been discussed for many years now.