GLib is the awesome standard library I get to use everywhere thanks to gobject-introspection. From work projects in Vala, window manager¹, image viewer², video player³. It is especially useful with lua configurable projects, given the sparsity of the language itself.

¹ https://awesomewm.org/

² https://github.com/muennich/sxiv - my user configs are lua & lgi, extended with gexiv2 also via gobject-introspection.

³ https://mpv.io/