Elasticsearch

Elasticsearch (https://www.elastic.co/products/elasticsearch) is a search and analytics engine based on Apache Lucene (http://lucene.apache.org/). It is technically a NoSQL database but it is primarily used as a search engine due to its indexing capability and high performance. It can be distributed and multi-tenant with full-text search capability. It has a web interface and JSON documents. It is one of the most used search engines.