ActiveWidgets.com - A component library that gives you AJAX-style fuctions with common visual elements.
Agile Partners Photo Resizing Tutorial - A tutorial on how to use the Prototype and script.aculo.us libraries to build an interactive photo resizer.
Drag & Drop Sortable Lists - Tutorial for creating sortable lists with drag & drop functions.
LightWindow - Another overlay window system that allows you to do it with multiple media file types and even other websites.
Prototype Windows Class - Allows you to add an overlay window to your HTML.
ThickBox - Works with jQuery to provide an AJAX hybrid for image overlays.