Xapian is an Open Source Probabilistic Information Retrieval library, released under the GPL. It's written in C++, with bindings to allow use from other languages. Xapian is designed to be a highly adaptable toolkit to allow developers to easily add advanced indexing and search facilities to their own applications. Xapian features include: - Ranked probabilistic search - word importance weighting - Relevance feedback - Phrase and proximity searching - Full range of structured boolean search operators - Term stemming - Database files > 2GB - Platform independent data formats - Allows simultaneous update and searching.