Tracker is a first class object database, extensible tag/metadata database, search tool and indexer.
It can trawl through your hard drive and index existing files and data stores
It has been designed from the ground up to be very lightweight (the tracker daemon consumes ~4MB of RAM in typical use) yet at the same time very fast too.
It provides a comprehensive, persistent and extensible storage system that can store and index almost any object. These objects can also have extensible user defined metadata and tags to create rich first class objects.