Tracker is a powerful desktop-neutral first class object database, tag/metadata 
database, search tool and indexer. 

Tracker is also extremely fast and super efficient with your systems memory when
compared with some other competing frameworks and is by far the fastest and most
memory efficient Nautilus search and Deskbar backends currently availble.

It consists of a common object database that allows entities to have an almost
infinte number of properties, metadata (both embedded/harvested as well as user
definable), a comprehensive database of keywords/tags and links to other
entities.

WWW: http://www.tracker-project.org/