I’m not going to suggest a plugin. Just some simple code to put on your sidebar.php
Let’s say, on your sidebar, you want to show only a particular category and its sub-categories but when you try to use widget, it does not let you choose which mother category you can set it to.
<?php wp_list_categories('orderby=name&show_count=1&child_of=3&title_li=<h2>Movies</h2>'); ?>
You may refer to TEMPLATE TAGS /WP LIST CATEGORIES for a list of options how you want your category list to appear.
But then it’s up to you to know how or where to paste it on your sidebar. 🙂