It is based on the very strong Xapian back-end, for which it provides a feature-rich yet easy to use front-end with a QT graphical interface.

Recoll is free, open source, and licensed under the GPL.

Features:

* Easy installation, few dependancies. No database daemon, web server, desktop environment or exotic language necessary.
* Will run on most unix-based systems
* Qt-based GUI. Can use either Qt 4 or Qt 5.
* Searches most common document types, emails and their attachments.
* Powerful query facilities, with boolean searches, phrases, proximity, wildcards, filter on file types and directory tree.
* Multi-language and multi-character set with Unicode based internals.
* (more detail)

http://www.lesbonscomptes.com/recoll/