Lucene

Driven by the Apache Foundation, the Lucene project is the solution selected by Wikipedia, among others, to index and perform searches on its content. Lucene is, without a doubt, the most well known, most widely used and most dynamic search tool on the open source market. It was released for free downloading by Doug Cutting in March 2000.

First and foremost, Lucene is a content search and indexing library. Like most search engines, Lucene is based on the concept of automated indexing, in other words, processing input data all at once and assigning them multiple links. On the functional side, Lucene supports searches for related forms of the same word (plural or singular, conjugations, tenses, etc.), synonym management, configurable relevance and more. It handles all of this with an exceptional level of performance.
Initially distributed under the LGPL, Lucene is now available under the Apache License.
Lucene is written in Java. It can be integrated with Java applications but also with applications in other languages like Ruby, Perl, PHP, C++ and more.