RethinkDB is an open source NoSQL JSON database designed for web apps. Its main differentiator is that you can tell it to continuously push updated query results to applications in real time, rather than having your app poll for changes. 

Elasticsearch replaced Solr as the most popular search engine 

At the moment, there are at least 35,000 publicly available, unauthenticated instances of MongoDB running on the Internet. 

MongoDB, one of the leading NoSQL databases, is well known for its fast performance, flexible schema, scalability and great indexing capabilities. At the core of this fast performance lies MongoDB indexes, which support efficient execution of queries by avoiding full-collection scans and hence limiting the number of documents MongoDB searches.

JSON support has only been introduced in MySQL 5.7.8, but it is undoubtedly the feature that arouses most enthusiasm among MySQL users.