Ajax - Sitepoint.com : JavaScript World Cup of Frameworks

And the Winners (for SitePoint) are:

• Dojo, a powerful library developed primarily off the back of JotSpot
• Prototype, the backbone of Ruby on Rails excellent AJAX support
• Mochikit, the Python library that makes JavaScript suck less
• Yahoo UI Library (often shortened to just YUI), the new kid on the block

Ajaxian gives a look on SitePoint article » JavaScript World Cup of Frameworks

Personnellement je parierais entre Yahoo UI Library qui va essayer d'imposer son look and feel par le biais de ses services grand public et Dojo Toolkit qui a le soutien d'IBM (et de Sun ?) et donc suivra au plus prêt les consignes Open Ajax