I created YUI 2 login widget, imYUILogin, for an intranet project that I am working on. I originally created it as a Dialog Container, but I recently updated it so that it could be configured as a Panel or a Module. Logging in is done ajaxally, of course.
I created a YUI widget that is based on my imAnimTabber JQuery Plugin. The imSliderMenu displays content by animating the height or the width of the contents container. I use the imSliderMenu widget on my marketing site, http://1st-steps-to-success.com. The slider menu is handy in that I am able hide content on a page until it is needed and still maintain the look and feel of my website design. As with the imAnimTabber JQuery plugin, The imSliderMenu dynamically creates the menu items based on CSS and a few lines of code.