JQuery is my favourite javascript library. It is lean and mean, pragmatic and generally a joy to use. The nicely organized web site and tons of easy to follow tutorials help as well. Moreover, JQuery fits nicely in a Nitro web application. I use a simple Nitro Element to integrate JQuery code in my templates:

class JQuery
    def render
        %~
        <script type="text/javascript">
            $(function() {
                #{content}
            });
        </script>
        ~
    end
end

And this is used like this:

<JQuery>
    $('#clickme').click(function(e) {
        $(this).toggle();
    })
</JQuery>

...

<div id="clickme">Hello</div>

Simple and effective, don't you think?