Grails ships out of the box with adaptive AJAX support.
By default Grails uses the Prototpe library when invoking AJAX request using the Grails remoteLink, formRemote and submitToRemote AJAX tags.
Not only is Grails providing an easy way of integrating AJAX functionality in your pages, but it also allows you to switch to another AJAX library if needed. This is what we mean with adaptive AJAX support.
One of the use cases to switch to another AJAX library is that this library offers more functionality then the default Prototype library. E.g. you want to use the Yahoo! User Interface Library (YUI) within you application for the autocomplete or calendar functionality. In this case you want the YUI library also to be used for your AJAX tags, so you only depend on YUI and not on both YUI and Prototype.
If you want easy support for YUI in your Grails application then just install the Grails Yahoo! UI Library Plugin which I released today. It downloads and installs automatically the latest YUI
For a personal application I want to use the YUI autocomplete and calendar functionality so I'm thinking already of a Grails YUI Widgets Plugin which can be installed on top of the YUI Plugin ...