Apache Lucene - High-performance, full-featured text search engine library written entirely in Java.
List.js - Tiny, invisible and simple, yet powerful and incredibly fast vanilla JavaScript that adds search...
Six Degrees of Wikipedia - Find the shortest path between two subjects
OpenSearch - OpenSearch is a community-driven, open source search and analytics suite derived from Apache 2.0 licensed Elasticsearch 7.10.2 & Kibana 7.10.2. It consists of a search engine daemon, and a visualization and user interface, OpenSearch Dashboards.
Sphinx Search - Sphinx is an open source full text search server, designed with performance, relevance (search quality), and integration simplicity in mind. Sphinx lets you either batch index and search data stored in files, an SQL database, NoSQL storage.
Butter.ai - Your team has Q’s, Butter.ai has A’s.