WebAlbums is a web-application for managing collections for pictures. It's my weekend project since more than four years, and currently holds some 4000 photos, 300 tags, 500 albums and 9 themes.
Photographs are naturally organised into daily albums grouped into themes (ie, collections). Then pictures' content is described with tags. Tags allows to group, and search for, pictures sharing common aspects. Gpx files can also be attached to albums.