If you're building a modern website then you'll be needing some javascript libraries and css.
Rather than hosting these common libraries on your own server, you should Use a Content Delivery Network.
Lucky for you Google, Microsoft, Yahoo and AOL host a range of popular javascript and css which you can directly link to for free.
This saves your bandwidth and speeds up your website load time.
CDN Catalog indexes these resources for your convenience.
http://ajax.googleapis.com/ajax/libs/jqueryui/1.8.1/jquery-ui.min.js
Also check out the css themes for jQuery UI.
http://ajax.microsoft.com/ajax/jQuery.Validate/1.7/jQuery.Validate.min.js
Extras: additional-methods.js
http://ajax.googleapis.com/ajax/libs/prototype/1.6.1.0/prototype.js
http://ajax.googleapis.com/ajax/libs/scriptaculous/1.8.3/scriptaculous.js
http://yui.yahooapis.com/2.8.0r4/build/yuiloader/yuiloader-min.js
http://ajax.googleapis.com/ajax/libs/yui/2.8.0r4/build/yuiloader/yuiloader-min.js
Also, you'll want to use the very awesome YUI 2: Dependency Configurator.
http://yui.yahooapis.com/3.0.0/build/yui/yui-min.js
Check out the YUI3: Dependency Congfigurator.
http://ajax.googleapis.com/ajax/libs/ext-core/3.1.0/ext-core.js
http://ajax.googleapis.com/ajax/libs/chrome-frame/1.0.2/CFInstall.min.js
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