I'm looking for open databases representing a tree data structure. I've already found the animal taxonomy, which is a perfect use case, but I'm looking for further open databases, focusing on a topic of commercial and/or scientific interest. I've found several databases representing a directed graph, but I have difficulty at finding databases representing a tree.
Check Wikipedia Category Tree
Wikidata Taxonomy. There is an hierarchy of "Instance of" P31 properties for each entity defined in Wikidata
On Wikipedia you can see both proprietary and open source graph database. The open source graph databases: Neo4j, RedisGraph, TerminusDB, JanusGraph, Grakn Core.
On the website of Apache project TinkerPop, you can also see a longer list of graph databases (both open source and proprietary) it supports.