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.
