Recently after looking at some of the plugin’s available I noticed some people including their own version of jquery or prototype. This is not how the plugin API was intended. WordPress comes with a bunch of useful scripts you can use without having to include in your plugin directly. The function is called wp_enqueue_script(). Here is the usage.

<?php

wp_enqueue_script(
     $handle
    ,$src
    ,$deps
    ,$ver
    ,$in_footer
);

?>

http://codex.wordpress.org/Function_Reference/wp_enqueue_script

So to include a copy of JQuery into your plugin all you have to do is add the following to your plugin.

<?php
	wp_enqueue_script('jquery');
?>

This tells WordPress to include jquery and will only include it if it hasn’t been included already which leads to a lower code footprint and better compatibility!

Of course your going to want to include that into wp_head. Here is the full code including the wp_head hook.

<?php
//function. Notice !is_admin this tells it to only include if its not admin
function sp_client_upload_init() {

       if (!is_admin()) {
	wp_enqueue_script('jquery');
        }
}
//hook to insert javascript into header
add_action('wp_head', 'sp_client_upload_load_css');	

?>

Here are the javascript files that are included with WordPress by default