I'm working on a project for which we're looking for some image gallery management software (ie, upload, organize, tag, etc.) that's a standalone piece of software: ie, not part of a larger CMS like WordPress, Drupal, ContentDM, etc.
We'd of course like something that is simple, awesome, and free. (We might settle for two of the three...) The only absolute requirement is that it needs to run on a Linux/Apache server. PHP/MySQL would be preferred, and something with a responsive front-end would also be nice.
The world appears to be teeming with such things of various quality; I thought I'd ask you all for recommendations rather than just try to wade through the masses of junk.