Apache Lucene

Apache Lucene is a high-performance, full-featured text search engine library written entirely in Java. It is a technology suitable for nearly any application that requires full-text search, especially cross-platform.

Official website of Lucene is http://jakarta.apache.org/lucene

Lucene can be downloaded from http://www.apache.org/dyn/closer.cgi/lucene/java/

A link with Lucene installation issues is available at http://www.gossamer-threads.com/lists/lucene/general/28710

Another nice link about how to use Lucene is available at http://www.javaranch.com/journal/2004/04/Lucene.html

The PoweredBy list of Lucene explains how powerful and useful it is.
http://wiki.apache.org/lucene-java/PoweredBy