Are there any databases which have a query language similar to RethinkDB's ReQL?
Many use SQL, while Mongo uses an 'object' API, and others use a REST API.
ReQL's core characteristic is that it embeds into native programming languages. This makes it a particularly 'natural' experience, proving very flexible with a very large range of capabilities and commands. Once composed, queries are then executed entirely on database servers.
So far, I'm aware of one similar database, FaunaDB. One notable difference in its query language is its use of UpperCamalCase. This might be jarring for JavaScript developers used to lowerCamalCase.