NextGEN Gallery is the most popular WordPress gallery plugin, and one of the most popular WordPress plugins of any kind, with over 6 million downloads.
It provides a powerful engine for uploading and managing galleries of images, with the ability to batch upload, import meta data, add/delete/rearrange/sort images, edit thumbnails, group galleries into albums, and more. It also provides two front-end display styles (slideshows and thumbnail galleries), both of which come with a wide array of options for controlling size, style, timing, transitions, controls, lightbox effects, and more.
The NextGEN Gallery WordPress plugin is now proudly maintained by Photocrati Media. Special thanks to Alex Rabe who created and maintained NextGEN from 2007 through 2011.