@Tgr pointed out on a recent internal mailing list thread that we have several pretty powerful/useful programming libraries that have been developed in such a way that they can be used by anyone. There was a blog post way back in 2015 about changes that were made to MediaWiki to make splitting out reusable libraries possible, but we haven't really done any follow up since then.
Here are some PHP libraries that may be worth highlighting:
I bet there are other things that I'm not aware of as well. Showcasing projects that have been around for a while would be reasonable too, like: