AskTaimoor

All this plugin does is that it adds ‘async’ in all script tags it find on the page. 
This helps in loading the webpage much faster.

Async Javascript Optimizer

Adds async to all javascript links for fast loading of webpage.

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

Async Javascript Optimizer vs Asynchronous Javascript