xapian.org
Xapian is an open source search engine library released under the GPL v2+ and written in C++, with bindings that allow use from Perl, Python, PHP, Java, Ruby and other languages. It is a toolkit that lets developers add indexing and search facilities to their own applications, with built-in support for multiple weighting model families and boolean query operators. Omega is a supplied search application built on Xapian.