Modernizr adds classes to the <html> element which allow you to target specific browser functionality in your stylesheet. You don't actually need to write any Javascript to use it.
When it comes time to deploy your new web application, instead of sending down a dozen javascript and css files full of formatting and comments, this Rails plugin makes it simple to merge and compress JavaScript and CSS down into one or more files, increasing speed and saving bandwidth.
Scriptaculous, MooTools, jQuery and much more: all you need to develop modern web interfaces and add new features to your website.
