CLucene

Lucene is an search and index library.
This giant from Apache and Dough Cutting is an freeware.This means we can use it in any of our application under Apache License.

Now what Lucene is a big question.
An Information storage apalication like news sites, job sites, serach engines, have loads of information to be mangaed and needs to give the right information on demand.
This is where Lucene helps.

Lucene provides a rich set of API for indexing your information and then search thru the information for the result.What adds more is the library comes in variety of langs i.e intially coded in Java , now has C++ named CLucene ,perl and a Ruby named Ferret version as well.So in what lang ur application is no matter.

if you want to see some the web application built on this library visit
nutch.com an Search Engine
CNet.com - an information bank of news
bixee.com - an Indian job site