Choosing your AJAX framework

There are hundreds of Ajax libraries and frameworks out there. Sébastien Just has given few guidelines here.
"
server independant or not ?
structural Javascript enhancements ?
re-usability of your written components ?
framework current documentation level ?
features you need ?
How long will it last ?
What sort of support ?
How steep is the learning curve for the framework?
Who are my visitors ? "

Additionally, one could consider points like
Cross browser support,
Access to the source code and its quality,
Flexible enough to support easy unit testing and javascript debugging,
Licensing.

For comparison of various web application frameworks, check out Wikipedia .

There is something in the AIR!

No, I'm not talking about MacBook Air! Adobe recently launched Adobe Integrated Runtime, or AIR, they also released Flex 3.0.
AIR is "a cross-operating system service oriented runtime that allows developers to leverage their existing web development skills in HTML, AJAX, XML, Flash and Flex to build Rich Internet Applications(RIA) to the desktop."
With AIR, Adobe is targeting AJAX developers to develop applications with AIR, not just Flash developers. Since it combines qualities of the web with a presence on the desktop, AIR gives opportunities to build a new range of more interesting Internet connected applications that run outside the browser. AIR supports most of the major AJAX frameworks including jQuery, Ext JS, Dojo, and Spry. It is based on WebKit HTML engine. Click here to see the list of features for AJAX development.
So users get additional desktop functionalities like starting AIR application from desktop, clipboard access etc. AIR also provides access to local file system. It provides seamless install process for AIR applications and also provides APIs that make application updates easy.
You can check out a list of featured applications that are developed on AIR.