Shows the child pages of the current page. Configurable title, show siblings (or not), and "no child" text
This plugin provides a "Sub Page" widget. It lists all of the pages which are children of the current page. If there are no children, it will not display.
While this is a small widget, I have attempted to make this as configurable as possible, with clean and efficient code (this is a small widget, and shouldn't slow your site down).