Meteora is set of cross-browser widgets and controls for quickly creating rich and customizable web applications without programming excessive JavaScript that is painful to debug in every browser.
Control Suite is a set of 6 high quality widgets and controls built with the Prototype JavaScript framework. Each script is well tested, highly extensible and unobstrusive.
Google Web Elements is a fresh website where you can find all Google widgets in one place & easily add any of them to your websites. The website simply offers a similar customization interface for each widget and generates the JavaScript code for embedding them. It currently presents 8 widgets.

jQuery-Accessible-RIA is built using the jQuery UI & works out-of-the-box. Also, they can be highly customized.

For older screen readers, it includes a workaround & for anyone willing to dive into the code, it is fully-commented
 

Requirements: jQuery 1.3.2+ & jQuery UI 1.7.1+

jTweetsAnywhere is a jQuery Twitter widget for easily integrating tweets into any website.
It supports Twitter's @Anywhere features and can display:

tweets from one or more user's feeds
tweets from a user's list
results of a Twitter search

It also

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