Sphinx is an open source full text search server, designed from the
ground up with performance, relevance (aka search quality), and
integration simplicity in mind. It's written in C++ and works on Linux
(RedHat, Ubuntu, etc), Windows, MacOS, Solaris, FreeBSD, and a few
other systems.

Sphinx lets you either batch index and search data stored in an SQL
database, NoSQL storage, or just files quickly easily and or index and
search data on the fly, working with Sphinx pretty much as with a
database server.

A variety of text processing features enable fine-tuning Sphinx for
your particular application requirements, and a number of relevance
functions ensures you can tweak search quality as well.

Searching via SphinxAPI is as simple as 3 lines of code, and querying
via SphinxQL is even simpler, with search queries expressed in good
old SQL.

Sphinx clusters scale up to billions of documents and tens of millions
search queries per day, powering top websites such as Craigslist,
DailyMotion, NetLog, etc.

And last but not least, it's open-sourced under GPLv2, and the
community edition is free to use.

WWW: http://sphinxsearch.com/