Smart Lists, a light-weight (8KB) Prototype-script.aculo.us framework extension, converts HTML lists into categorized, paginated lists. And, It does not require a database or server-side script.
It can be applied to existing lists with very little effort, providing a significant bump in usability.

Smart List is easily configurable with different hide/show effects for items and changing pages, speed of the effects and number of the items for paging.
Besides supporting all the major modern browsers, it degrades gracefully for unsupported browsers.
ProtoFlow is an open source content viewing application built with Prototype & script.aculo.us which functions similar to the famous iTunes Cover Flow.
You can browse through the content (it supports all the HTML elements) with mouse clicks or a JavaScript slider.

It is mentioned that the application may have more features in the future like auto-play and image reflection.
Online image cropping is a widely used feature in web applications.
Prototype JavaScript Image Cropper UI is an unobtrusive script based on Prototype and script.aculo.us.
It allows you to select any area in an image and gives the x, y, x1, y1, width and height values which are enough for a succesful image cropping.

This Prototype image crop UI also has ratio locking, selected area preview and minimum height - width options.
Requirements: Prototype v. 1.5.0_rc0+, script.aculo.us v. 1.6.1+
Compatibility: Chack out
Known issues
ModalBox is a JavaScript technique for creating Web 2.0 - style modal dialogs (or sequence of dialogs) without using standard popups and page reloads.
It’s inspired by Mac OS X modal dialogs and based on Prototype JavaScript framework + script.aculo.us and valid XHTML/CSS. Content can be loaded with Ajax or just plain-HTML.

ModalBox Features
- Web 2.0-ready. ModalBox uses industry-standard libraries — prototype and script.aculo.us.
- Offline-mode. Use dynamic- or plain-HTML without any Ajax-calls to fill out your dialog windows.
- AJAX page loading. ModalBox uses AJAX instead of deprecated
iframe for content loading. It’s also more secure — you can’t access pages that are not on your host.
- Callbacks support. You can attach your own JavaScript events after showing or hiding (and more) the ModalBox
- Automatic height adjustment. ModalBox adjust it’s height depending on your content. No more height tweaking!
- Scrolling mode. If your content is long, just define the height of the ModalBox and it will switch into “scrolling mode”
- Browser and platform independent. Since most modern browsers use popup blockers, it’s hard to find another way to create 100% browser-compatible modal dialogs.
- Multi-purpose. You can create complex wizards to guide users through the process. Image slideshows can be created too.
- Keystrokes support. Use ESC key to close ModalBox.
- Customizable Look & Feel. Use CSS to make ModalBox look like you want.
- Supports transitional effects. Slide down appearing and on-the-fly resizing.
- Lightweight. Just about 10 KB of code.
- Works in most modern browsers. Tested in IE6, IE7, Firefox 1.0, 1.5, 2.0, Safari, Camino, Opera 8 and 9.
P.S. There is a Mootools clone of ModalBox named MOOdalBox.
Requirements: Prototype + script.aculo.us (included)
Compatibility: All Major Browsers
Starbox is a star rating library built on Prototype Javascript framework which you can use to create most types of star rating systems with 1 png image.
In order to add some extra effects to the star rating code, you can add script.aculo.us as well (sample codes can be found in the Starbox website).

Some features of Starbox:
- Re-rating allows you to adjust your previous rating.
- Easily saving your rating with Ajax using the build in callback.
- Styling the Starbox based on status (hover, rated and locked).
- Good seperation of Javascript, CSS and images.
- A ghosting bar that tracks your average.
- Batched building of Starboxes based on cached overlay images.
Requirements: Prototype 1.6+, script.aculo.us for effects.
Compatibility: All Major Browsers