Jquery.lazyload is a project mainly written in ..., it's free.
Lazy Load - jQuery plugin for lazy loading images
Lazy Load - jQuery plugin for lazy loading images
Copyright (c) 2007-2009 Mika Tuupola
Licensed under the MIT license: http://www.opensource.org/licenses/mit-license.php
Project home: http://www.appelsiini.net/projects/lazyload
Version: 1.5.0
UPDATE 11 Oct 2011: Cyril (www.yatahonga.com) With the update it really delays the image loading from server (no HTTP request before appearing) and HTML code remains XHTML 1.0 / 1.1 valid
Usage: src: your "preloading" image title: URL of the image
Example: