SQL
https://www.mysql.fr
https://mariadb.org
http://www.postgresqlfr.org

NoSQL
https://www.rethinkdb.com
http://couchdb.apache.org
https://pouchdb.com/
https://www.elastic.co/fr/products/elasticsearch
https://redis.io

RethinkDB howtos
https://pusher.com/tutorials/live-node-rethinkdb
https://developer.ibm.com/clouddataservices/2016/09/13/real-time-qa-app-with-rethinkdb/
https://keetmalin.wixsite.com/keetmalin/single-post/2018/08/02/Building-a-Real-time-Notification-System-with-RethinkDB

CouchDB howtos
https://rethinkdb.com/faq
https://news.ycombinator.com/item?id=10688447
https://www.twilio.com/blog/2012/09/building-a-real-time-sms-voting-app-part-1-node-js-couchdb.html

File & memory
https://gun.js.org
https://github.com/techfort/LokiJS
https://github.com/agershun/alasql
https://github.com/typicaljoe/taffydb
https://github.com/louischatriot/nedb

Big Data
http://cassandra.apache.org/
http://hbase.apache.org/
http://basho.com/products/riak-kv/

Time Series
https://www.influxdata.com/time-series-platform/influxdb/

Graph (orienDB is hybrid, neo4j focuses on graph)
http://orientdb.com/
https://neo4j.com/
https://lemag.sfeir.com/transformez-vos-donnees-csv-rdf/

Comparison bt. the two and general guide on graph DBs
https://www.quora.com/Is-there-any-reason-not-to-use-OrientDB
http://info.neotechnology.com/rs/773-GON-065/images/Definitive-Guide-Graph-Databases-for-RDBMS-Developer.pdf

Frontend DBs/API
https://hasura.io
https://directus.io
https://parse.com
https://supabase.io
https://www.8base.com
https://www.kinto-storage.org
https://pouchdb.com/
https://www.apollographql.com/client/
http://www.js-data.io
https://gun.js.org
https://www.dreamfactory.com # OSS (concurrent of APIgee, Mulesoft)
http://postgrest.org
https://g-ernaelsten.developpez.com/tutoriels/PostgREST/

Next Gen Serverless
https://fauna.com/
https://hasura.io
https://realm.io

Reviews and opinions
http://blog.nahurst.com/visual-guide-to-nosql-systems
https://www.baylon-industries.com/news/?p=1942
https://kkovacs.eu/cassandra-vs-mongodb-vs-couchdb-vs-redis
http://stackoverflow.com/a/37632625/1717735
https://hashnode.com/post/what-are-the-key-advantages-of-couchdb-over-other-nosql-offerings-ciitugn9g008poy53iixsc2d1
https://stackoverflow.com/questions/45730820/sql-server-and-firebase-pouchdb-synchronization