Obaid Ahmed has written a coverflow-like component on top of Prototype and Script.aculo.us called ProtoFlow.
Lightview 2: Not just images, now on Prototype

Lightview was built to change the way you overlay content on a website. * Clean: Designed to compliment your content. * Fast: Smart image preloading. * Easy to customize: You don’t even have to know CSS. * Rounded: Adjustable rounded corners, without PNGs. * Smart resizing: Content will always fit on your screen. * Custom [...]
Milkbox – Lightbox for Mootools 1.2

Another lightbox clone, but this is written for Mootools 1.2.
Accordion Menu script

A list based vertical menu with nested content that expand and contract when the user clicks on a menu header. Specify which menu headers are expandable, style the headers differently depending on whether its content is expanded or not, and specify whether persistence of the menu states within a browser session should be enabled. Clean [...]
Step Carousel Viewer
Step Carousel Viewer displays images or even rich HTML by side scrolling them left or right. Users can step to any specific panel on demand, or browse the gallery sequentially by stepping through x number of panels each time. Contents for the Step Carousel Viewer can be defined either directly inline on the page or [...]
Unobtrusive Slider Control V2

Whenever a slider has focus, the arrow keys ←, →, ↑ and ↓ can be used to control the slider handle, the Home key to set the slider at it’s minimum value and the End key to set the slider at it’s maximum value. The slider can be associated with both text input and select [...]
Creating a table with dynamically highlighted columns

How to create a table like Crazy Egg’s pricing table on their Pricing & Signup page. This tutorial will show you how to recreate the behavior step by step, and share the power of combining CSS, JavaScript, and images in clever ways.
Masked Input Plugin

A masked input plugin for the jQuery javascript library. It allows a user to more easily enter fixed width input where you would like them to enter the data in a certain format (dates,phone numbers, etc). It has been tested on Internet Explorer 6/7, Firefox 1.5/2, Safari, and Opera.
Custom Checkbox with jQuery

Provides you with the ability to customize the design of Checkboxes in your web forms. You can use the default skin and the Safari skin which are provided with the package.
LiveValidation

LiveValidation is a small open source javascript library for making client-side validation quick, easy, and powerful. It comprises of two main parts. Firstly, it provides developers with a rich set of core validation methods, which can also be used outside the context of forms. Secondly, it provides your visitors with real-time validation information as they [...]
Cross Browser Multi-Page Photograph Gallery

Hovering over the tabs changes the category, while hovering over an image enlarges it.

