Although I always use LuaLaTeX to compile my documents, I don't really use Lua that often. Sometimes I even forget about it.
In my last document I used it to write a random word generator, which picks a word from a pool of words. Which can be really helpful in many different documents and circumstances. I came up with this code in another question, but I will repost it here, if someone wants to use it.
So today I asked myself, what are some other good uses for Lua in TeX documents? And here we go, now it's your turn to post your ideas.
Thanks for all the great answers. There are some great ideas, and I certainly can put them to good use in my own documents (and other users aswell, I guess :) ). In my opinion, every answer deserves the check mark, so I guess I will let the community decide and choose the one with the most upvotes.
That of course doesn't mean you have to stop posting your awesome ideas ;)