MongoDB, Redis, PostgreSQL and Elasticsearch are amongst the top databases (most popular) according to StackOverflow's 2019 Developer Survey.
Redis is an in-memory store that supports strings (key / value), hashes (or maps, collections of key / value pairs), lists, sets etc.
It is really fast and pretty easy to learn.
Like MongoDB it is also non relational, but by nature, the database's size is limited to the amount of available memory (RAM).
As opposed to MongoDB, which is a Document Database, PostgreSQL is an Object Relational Database that uses a variant of SQL as the query language. As a relational databases, it performs well when it comes to storing information that relates to one another.
postgresql.org
PostgreSQL Tutorial
PostgreSQL Full Course for Beginners by freeCodeCamp
Another type of databases that performs really well when relationships between entities become more complex (and contain precious information) is the graph.