Xapian is a Search Engine Library which aims to be fast, scalable, and flexible. It's used by many organizations and software projects around the world, including Debian, Gnome, KDE, One Laptop per Child, and Ubuntu.

It supports ranking by Language Modelling, TF-IDF, probabilistic schemes, and Divergence from Randomness, plus a rich set of boolean query operators, and re-ranking using Machine Learning. The core library is written in C++, with bindings to allow use from many other languages.