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.