Apache Lucene - High-performance, full-featured text search engine library written entirely in Java.
Wikiwand - A beautiful new interface to Wikipedia
Six Degrees of Wikipedia - Find the shortest path between two subjects
Wikiverse - An interactive galaxy of Wikipedia knowledge ✨
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.
WikiLinks - Explore Wikipedia through a media-packed, mind-mapping experience.