phpGraphy is a free PHP photo gallery script released under GPL license. Since its beginning, it has been written with simplicity and efficiency in mind following the KISS philosophy.
It should fit many needs, thanks to its easy installation process and minimal requirements, it can be setup in less than 3 min to put a small photo gallery online. For users with thousands of pictures, don't worry as phpGraphy does use a real directory tree to store the pictures (as you probably already have on your hard-drive), you'll just have to upload your whole picture collection et voila, phpGraphy will generate the thumbnails and low-resolution pictures for you and even better, it can also auto-import IPTC description in the database (if already embeded in your pictures).
It integrates a bunch of nice features like comments, rating, EXIF/IPTC handling, go to the Features page for more details. If you want to see script in action, try the Online Demo. If you want to try-it now, go on the download page.