So, you want to use FontAwesome in your Theme, eh? Add the following snippet to your functions.php file:

wp_enqueue_style( 'font-awesome', '//netdna.bootstrapcdn.com/font-awesome/4.1.0/css/font-awesome.min.css' );

Do you want to use it in your Plugin? Just place that code in one of your functions within your plugin file and hook into the wp_enqueue_scripts Action:

add_action( 'wp_enqueue_scripts', 'plugin_slug_load_iconfont' );

function plugin_slug_load_iconfont() {
    wp_enqueue_style( 'font-awesome', '//netdna.bootstrapcdn.com/font-awesome/4.1.0/css/font-awesome.min.css' );
}

The same principle holds true for the other icon fonts that are available online.

But here, this one’s even easier: Dashicons. They’re already included as part of the WordPress admin dashboard, so in order for you to get them working for non-logged in users, all you have to do is:

add_action( 'wp_enqueue_scripts', 'plugin_slug_load_dashicons' );

function plugin_slug_load_dashicons() {
    wp_enqueue_style( 'dashicons' );
}

Simple, right?