Personally, I am a fan of MooTools ' animation classes ( Fx.Tween , Fx.Morph , Fx.Transitions ). Very simple and easy to use. For more advanced animations, Fx.Slide, Fx.Scroll and Fx.Elements are also available ...
It also has a neat Ajax (Request) class that takes care of all your ajax needs.
Obviously, although this is my personal opinion ... Any of the large ones (the user interface of Yahoo, jQuery, MooTools, Prototype, etc.) will be able to perform both Ajax and animation, so I would suggest looking at a sample code from all these libraries and choose the one you like best.
jeannicolas
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