Many internet users are storing their images on photo sharing/hosting services like Flickr, SmugMug or Picasa to make sure they are easily reachable and backed up regularly.
OpenPhoto is an open source PHP application that encourages users to host their images in the cloud (Amazon S3, Rackspace CloudFiles, Google Storage, etc.) on their own and functions as a front-end to manage + present them.
This way, images stay totally under control of users, switching providers become much easier and users won't be limited with the capabilities offered by photo sharing/hosting services.
The front-end has a simplistic interface where photos are displayed as a grid, categorized with tags and searched easily. It is also mobile-friendly and has theming support.
OpenPhoto itself is built on top of the powerful API which makes it possible to improve it or integrate photos with any other apps.