Uses HeadJS to load your enqueued scripts asynchronously, in parallel, executing them in order.

dmikam

Async JS and CSS

Converts render-blocking CSS and JS files into NON-render-blocking, improving performance of web page.

Paris Holley

Asynchronous Javascript

Improve page load performance by asynchronously loading javascript using head.js

kylereicks

Head.WP

Head.js is a script to asynchronously load and manage dependencies of javascript and CSS assets.

Brian Zeligson

This plugin uses the wp_print_scripts action hook, as opposed to output buffering and regex used by alternative implementations.
The downside of this method is that only scripts loaded via wp_enqueue_script will be affected by the plugin, the upside is better 
performance by avoiding output buffering on every page load.
The plugin will preserve any localizations added via wp_localize_script, and uses the first parameter passed to wp_enqueue_script 
as the label for the script in the head.js call. For example,
<pre><code>wp_enqueue_script('jquery', 'https://ajax.googleapis.com/ajax/libs/jquery/1.7.1/jquery.min.js');
</code></pre>
would show up as
<pre><code>head.js({"jquery": "https://ajax.googleapis.com/ajax/libs/jquery/1.7.1/jquery.min.js"});
</code></pre>
This allows you to run callbacks when specific scripts are ready, such as
<pre><code>head.ready('jquery', function() {
 //do something when jquery is loaded
});
</code></pre>
For more on HeadJS usage, see http://headjs.com/

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