prettyGallery is a jQuery plugin for creating sliding image galleries on the fly.
Besides images, any content can be presented within the slider.

Images in the gallery are defined in an ordered list. Items per page, speed of the navigation & labels used can be configured.
The gallery uses prettyPhoto, a jQuery Lightbox clone, for displaying the images in large-size.

This lightbox clone is very good-looking & can display:
- images
- present the images as an image gallery
- Flash
It works in all major browsers & animation speed, opacity and more settings can be updated.
Requirements: jQuery
Compatibility: All Major Browsers
This is a 3D Flash gallery built with Papervision 3D.
It uses a XML as the database for image information & displays the images with a nice effect.

You can check a demo of the gallery here.
The download package includes the source files which is good for improving it and/or taking a deeper look to the use of Papervision 3D.
Update: Tutorial on creating a Papervision 3D gallery can be found here.
Requirements: No Requirements
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.
Requirements: PHP
Compatibility: All Major Browsers
FlashFlickr PhotoGallery is an open source photo gallery built with Flex 2 / AS3.
The application enables users to browse your Flickr pictures, see your sets, tags and notes within your website from a good looking interface.

The gallery has a location map feature built with Yahoo AS3 libraries that shows the location of the image (if mentioned) using Yahoo maps. This requires a Yahoo API key.
It also has the Flickr authentication for Flickr registered users. This way, registered users can also add comments, tags (if allowed) to the images (Flickr API key needed).
Requirements: Flickr & Yahoo API keys.
Showcase is a Prototype extension that displays a "list of images" as a CoverFlow gallery.
And, besides the standard horizontal look, it works both vertical & diagonal.

The extension can be easily configured to display any number of images, durations of the animation can be updated & more.
Showcase requires the effects.js component of Script.aculo.us & works in all major browsers.
Requirements: Prototype & Effects.js component of Script.aculo.us.
Compatibility: All Major Browsers
SmoothGallery is a feature-rich image gallery & slideshow application built with MooTools.
With the "gallery set" function that enables you to present multiple galleries & different transition animations, it is really powerful.

SmoothGallery is designed from the ground up to be standard compliant: You can feed it from any document, using custom css selectors. It is also compatible with all major browsers.
And even better, this solutions is very lightweight: The javascript file is only 24kb.
Requirements: MooTools 1.11
Compatibility: All Major Browsers
Image Gallery is a free Flash image gallery that comes with the source code.
Settings are configured from a XML file like the names of the categories, images & folders where the images resides.

The gallery presents the images randomly or with an order (can be set via the XML file).
When a thumbnail is clicked, original image is presented in a Lightbox like Flash pop-up, images in that gallery can be browsed easily & even viewed as a slideshow.
The demo of the gallery can be viewed from here. As there is no direct link for the source file in the website, you can download it from here.
P.S. I have contacted Tony Yoo (owner of the gallery) & yes, it is free.
Requirements: No Requirements
Flashmo, which was presented at WRD (here & here) a while ago has 3 new free Flash galleries.

The galleries are:
Images in the galleries can be edited via a XML file
All the galleries have the source FLA files & you can edit them to make the galleries fit your needs.
Galleriffic is a jQuery plugin for creating fast-performing photo galleries.
Images can be viewed as a slideshow or by manually clicking the thumbnails.
The gallery has support for paging which makes it possible to display large amount of images.

Images in the gallery are mentioned as a JavaScript array and every image is loaded once they are called.
Galleriffic is also SEO friendly, you can directly view any image with the specific gallery URL of it.
Requirements: jQuery 1.2.6
Compatibility: All Major Browsers
UvumiTools has 4 great & free MooTools plugins that can save your development time.

This is an easy to use cross-browser crop interface that has options like:
- Setting the minimum size
- Keeping the aspect ratio
- Live preview of the area to be cropped
- Supports HTML resizing

This plugin enables you to create good-looking image galleries.
Users can click a thumbnail and watch the gallery transform into a slideshow, which is then navigated by clicking the left column or by scrolling with the mouse wheel. A click on the photo being displayed brings the user back to the contact sheet view.

This dock menu, similar to a Mac interface, is passive under the screen & gets activated when an item is mouse-overed.
The menu is created from a valid unordered list & can be applied within minutes.

A cross-browser, animated dropdown menu with unlimited levels support.
It is created from unordered lists & can be totally customized via CSS.
Requirements: MooTools
Compatibility: All Major Browsers