Apache Lucene - High-performance, full-featured text search engine library written entirely in Java.
Pathica - Find a path to connect with anyone on Instagram.
Six Degrees of Wikipedia - Find the shortest path between two subjects
Endless - Random and amazing Wikipedia articles.
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.
Data Dashboard by Wikimedia Foundation - Compiled data of the Worlds busiest public site