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!)
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.
Xapian and Omega