Piwigo is an open source photo gallery system built with PHP & MySQL. Piwigo is very flexible in means of adding photos to the system. They can be added via pLoader (the remote client of Piwigo), mass uploading by FTP or the application can call them from any other servers transparently.
With the unlimited category-levels support, images can be organized easily. And, with the ability of tagging them, it just gets better.
A great feature is chronology. Piwigo reads EXIF data, understands when an image is taken & shows them in a chronologic order.
The application has also multilanguage support, can produce pretty URLs & an API makes integrations with any 3rd party applications possible.
Homepage & Download: http://piwigo.org/