Xapian is a highly adaptable toolkit which allows developers to easily add advanced indexing and search facilities to their own applications. It has built-in support for several families of weighting models 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.