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.

kylereicks

Head.js is a script to asynchronously load and manage dependencies of javascript and CSS assets.
This plugin loads the scripts and styles enqueued in footer with the head.js loader. It does not include head.js’ feature detection.
Head.js.wp assumes that scripts are loaded via <code>wp_enqueue_script()</code> and that any script enqueued in the <code><head></code> intends to be loaded before the rest of the page.

Head.WP

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

Async Javascript Optimizer vs Head.WP