I've mostly experienced it through World of Warcraft -- it's embedded there as the language used to write the game's user interface[1]. Then it's exposed to third-party authors as an API, so they can write addons for the game.

It's also embedded in Wikipedia[2]. Complex templates can be written with Lua modules, rather than relying on wikitext.

[1]: http://wowwiki.wikia.com/wiki/World_of_Warcraft_API

[2]: https://www.mediawiki.org/wiki/Extension:Scribunto