Gallery2 is a highly featured web gallery. I use it across my web-sites here, embedded in both WordPress and Drupal.

It is a complex system, with a very abstracted implementation which is hard to fully understand. Out of the self-hosted galleries it is probably the most suitable for me, but I am oft-tempted to move away from it – Flickr seems tempting, but by the time I bought the features I need, and multiplied that by the number of users using my Gallery it is no longer quite so attractive.