Xapian Project – Open Source Search Engine Library
http://www.xapian.org/

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. This has been added to World Wide Web Reference Subject Tracer™ Information Blog. This has been added to the tools section of Research Resources Subject Tracer™ Information Blog.