Xapian is an Open Source Search Engine Library, released under the
GPL. It's written in C++, with bindings to allow use from Perl,
Python, PHP, Java, Tcl, C# and Ruby (so far!)

Xapian is a highly adaptable toolkit which allows developers to
easily add advanced indexing and search facilities to their own
applications. It supports the Probabilistic Information Retrieval
model and also supports a rich set of boolean query operators.

If you're after a packaged search engine for your website, you
should take a look at Omega: an application we supply built upon
Xapian. Unlike most other website search solutions, Xapian's
versatility allows you to extend Omega to meet your needs as they
grow.