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#, Ruby and Lua (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.
‒ Courtesy of Xapian
There are currently 4 groups being archived from Xapian (lists.xapian.org).