minishowcase is a free & amazing Ajaxed PHP image gallery.
It has multi-gallery support & does not require any database. Images uploaded are handled via a PHP file & auto-thumbnailed.
When a thumbnail is clicked, they are presented in a Lightbox. There is also a slideshow feature which can be easily configured.
The look of the gallery can be changed with its templating logic. And, it has multi-language support (English, French, Italian & Spanish already included).
Although the application uses Ajax, it generates permanent URLs that every image & gallery has its own URL.
There is also a free desktop application named Mini Gallery Manager which enables you to bulk-upload, resize, rotate images & send them to the gallery.
Compatibility: All Major Browsers