Apache Lucene - High-performance, full-featured text search engine library written entirely in Java.
Conrad Wise - Chat with the biggest brain on the planet!
Six Degrees of Wikipedia - Find the shortest path between two subjects
botwick - Build your own SMS-based text bot
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.
Peek by Sensay - Peek into real chats happening in messengers