PUBLIC MARKS with tags flat-hierarchy & license:gpl

GLS³ is an open source semantic storage solution for GNU/Linux that indexes your data, extracts from it metadata and relevant information, allows you to organize it using queries and tags, an API to allow Developers to integrate searching and organization capabilities in their application, an extensible plugin-based Type System, shared schemas between applications through an API, a pseudo file system for backward compatibility, a web interface, As-You-Type searching and more.

digiKam is a simple digital photo management application for KDE, which makes importing and organizing digital photos a "snap".