Wikidata, SPARQL, and RDF in general. And I guess semi-relatedly things like Prolog? I recently decided to fiddle with Wikidata, and it is fascinating to be able to query for any part of a statement, not just the value! In SPARQL you write statements in the form But the cool part is that any of those three parts can be extracted, so you can ask things like "what are the cities in ", or "what country harbors the city…
I've built a few solutions with graph databases (cypher is my query language preference, oddly) and use an RDF/OWL ontology for some personal documentation projects, and what I think holds it back in production is it is too powerful. "Too powerful" doesn't seem like a thing until you realize it undermines DBA's skill investments, means business level people have to learn something and solve their own problems instead…
I'm interested in this field and find it fascinating but we're still in it's early dark ages.
when it comes to Knowledge representation and reasoning there's too much emphasis on the representation part and less on the reasoning part, but even this representation part is not a solved problem.