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.

WWW: http://www.xapian.org/