Autover

Autover plugin will automatically version your CSS and JS files. This is especially useful when using a Content Distribution Network (CDN) for delivering static files like CSS and JavaScripts. The plugin appends to the resource URL a numeric version number generated by the filemtime() PHP function (e.g. style.css?ver=1359464809). This means the version number will be updated every time the CSS or JS file is being saved.

IMPORTANT: this plugin only works with CSS and JS files that are enqueued using the standard WordPress functions wp_enqueue_style and wp_enqueue_script and/or admin_enqueue_style, admin_enqueue_script.