Validatious is an unobtrusive & easy to use JavaScript form validation library that does not depend on any JS frameworks.
It has built-in validators like numeric, e-mail, URL, max/min char, etc. & custom ones can be created.

Using the library, it is possible to create chained validation rules like: validate( "email".is("required").andIsAn("email") );.
How and when Validatious does validation, how it displays errors & more can be controlled.
The library supports multilingual error messages with the help of a language file or overriding pre-built ones.
Requirements: No Requirements
Compatibility: All Major Browsers
Gaya Design is featuring a very nice tutorial (with source) on creating an image slider with a progress bar.
The slider uses the powerful jQuery Cycle plugin (see WRD post) which can rotate images with various effects.

The progress bar is created automatically to reflect the number of images that wi,ll be displayed.
Besides the look & feel of the slider, variables like speed of the slideshow & transitions can be set too.
Here are several examples provided to better findout how it functions.
Requirements: jQuery, jQuery Cycle Plugin
Compatibility: All Major Browsers
Thematic maps are very useful when describing statistical data about places. The data, simply, becomes much easier to understand.
Cartographer is a JavaScript library for creating thematic maps on Google Maps which supports custom styling.
It is only a one JavaScript file that relies on the powerful Raphael.js vector-graphics library.

It supports 3 visualizing types:
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Chloropleth (color coded regions)
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Pie charts
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Point clusters
The areas on maps are mentioned as latitudes or regions (depending on the type) which means no hacking or manual calculations.
Every visualization type comes with different options like colors, opacity, stroke & more.
P.S. The library is currently on alpha state but pretty solid.
Requirements: RaphaelJS JavaScript Library
Compatibility: All Major Browsers
Mu is a JavaScript library for enabling Facebook Connect on any website with ease.
It makes accessing the following possible:
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Identity: the user's name, photo and more
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Social Graph: the user's friends and connections
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Distribution: the Stream, and the ability to communicate
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Integration: publishers, canvas pages, profile boxes & tabs

Mu is lightweight can be used with other libraries with no conflicts.
There is also a console provided for experimenting the Mu APIs.
Requirements: No Requirements
Script.aculo.us is the most widely used add-on for the Prototype JavaScript Framework with the features like animations, drag'n drop, Ajax control & more.
Its second version, renamed as Scripty2, is out with a complete rewrite. It is lightweight (5kb compressed), flexible & works faster.

The library is currently in alpha status & there are still parts that need to be developed or improved. On the other hand, it is very promising & already being used by several big projects.
And, besides the Scripty2 homepage itself, the demos show how it will be empowering Prototype JS.
Requirements: Prototype JS
Compatibility: All Major Browsers
Floom is a MooTools slideshow plugin that can present images & their captions beautifully.
It displays them with a blind-effect & a chic loader that shows when the next image will be shown.

The plugin has various options like:
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defining the number of blinds between transitions
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slideshow interval
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loader on/off
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captions on/off
To mention, Floom is not a complicated script, rather, it is a lightweight (4kb compressed) one with very few features (just auto-slides images) which is sometimes "what is needed".
P.S. The download link in the website, currently, has a typo. You can use this address until it is fixed.
Requirements: MooTools
Compatibility: All Major Browsers
eCSStender is an open source JavaScript library, created by Aaron Gustafson, that helps browsers (including IE6) pay attention to the CSS code you write.
It aims to prevent the usage of browser-specific properties & hacks but make leaner & meaner CSS possible.

eCSStender is an extendible library with its API support & has ready-to-use extensions like CSS3 selectors, @font-face, rotation & more.
How does it work?
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it loops through the document's stylesheets and begins collecting information
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parses the rules it understands
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checks if the parsed rules are supported by the extensions
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applies the supported rules to the document via JavaScript
Read the rest of this entry »
Calender Eightysix is an unobtrusive calendar & datepicker that is built with MooTools.
It offers a very easy-to-use navigation for jumping back / forth between months, years & decades without drop-down boxes.

The script is lightweight (9.5kb) & can be themed via CSS (3 themes are already included).
It has a bunch of options for maximum customization like setting:
- default date on load
- min-max dates of the calendar
- date format
- unpickable days/weeks
- durations between slides, fades, toggles
- the calendar start date (Monday – Sunday)
- & more..
P.S. It is also the successor of the popular Vista-like Ajax Calendar (WRD post).
Requirements: MooTools, MooTools More
Compatibility: All Major Browsers
Ad Gallery is a jQuery plugin for creating an image gallery/showcase quickly.
The plugin uses unordered lists & converts them by offering lots of options for a better customization.

Some features of the plugin are:
- auto-start from any image
- keyboard navigation support
- custom effects like slide, fade..
- show image descriptions
- clicking the images close to the next-prev buttons will call more thumbnails
- work as a slideshow with start-stop buttons
- & more..
Ad Gallery preloads the images for a flawless display, however, if the image to be shown is not preloaded, it displays a loader & waits for the image to load.
And, depending on the image size to be presented, it auto-arranges the dimensions of the "container div" for a better fit.
Requirements: jQuery
Compatibility: All Major Browsers
Anyone who visited Mashable lately, the popular social news website, might have seen the "drag to share" functionality which appears when hovering an image.
It is a smart technique which encourages visitors to share a story by offering a quite fun usage.

Nettuts+ is sharing a very nice tutorial (with source) that enables you to implement this feature using jQuery & jQuery UI.
It makes use of the draggable & droppable components of jQuery UI to make drag’n dropping & targeting objects possible.
To see how it works, a demo is provided as well.
Requirements: jQuery & jQuery UI
Compatibility: All Major Browsers