Addings WordPress shortcodes to custom widgets

To add plugin shortcode to some text area of a widget that you’ve built, follow these three steps. Keep your eye on the my_content tag.

1. Make sure that the variable that you want to add the short code to has apply_filters applied to it.
   $content = apply_filters('my_content', $instance['content']);
2. Throw the following two lines into your functions.php file.
   add_filter( 'my_content', 'shortcode_unautop');
add_filter('my_content', 'do_shortcode');
3. Add the shortcode to your widget.

Steps two and three come from http://digwp.com/2010/03/shortcodes-in-widgets/.
Step one was sort of inferred from http://codex.wordpress.org/Function_Reference/do_shortcode