Paris Holley

This plugin is meant to be a drop-in to your wordpress installation with no additional configuration. The goals/features of this plugin are:
<ul>
<li>Load javascript files in an asynchronous manner to improve time to DomReady/OnLoad</li>
<li>Use existing wordpress APIs for backwards compatability and prevent coupling to this plugin</li>
<li>Leverage dependency model wordpress provides for assets to improve loading performance</li>
</ul>
Please submit bugs or contributions to the github location and not here on wordpress’ system:
https://github.com/parisholley/wordpress-asynchronous-javascript/
SEO: async js, asynchronous js, async javascript
<h3>Incompatibility</h3>
This plugin will not work out the box with the following plugins (unless they are modified to support asynchronous loading).
<ul>
<li>Jetpack by WordPress.com (social plugin)</li>
<li>WP Most Popular</li>
</ul>

Asynchronous Javascript

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

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/

WP HeadJS

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

Asynchronous Javascript vs WP HeadJS