What does this have to do with jQuery?

You can make whatever plugin you want that has jQuery in it. Commercial or not.

You can make a WP plugin that uses jQuery, in fact, you can do:

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

And not even have to include jQuery yourself, it uses the jQuery bundled with WordPress.

Just make sure you use your selectors like jQuery('something') instead of $('something') to avoid conflicts with scriptaculous.