Apache Lucene - High-performance, full-featured text search engine library written entirely in Java.
Wikipedia - Wikipedia is a free online encyclopedia, created and edited by volunteers around the world and hosted by the Wikimedia Foundation.
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.
Miraheze - Miraheze is a wiki farm (hosts wikis) for free and with no ads, it also provides custom domains...
PIPL Professional People Search Engine - PIPL professional search is used to find the person behind an email,phone or social profile and covers over 3 Billion people worldwide.
Kiwix - Kiwix enables you to have the whole Wikipedia at hand wherever you go!