insightdes

A widget to display a vertical menu which contains the current page’s parent menu and all sibling pages.
Can be styled with CSS in your theme.

Parent Page Menu

A widget to display a vertical menu which contains the current page's parent menu and all sibling pages.

littlejon

LJ Subpages Widget allows you to display a menu listing subpages from a chosen page.
This plugin was developed as I needed a widget to display a list of subpages in my sidebar. All the other plugins that I found would only display subpages from the current page being displayed.
Features:
<ul>
<li>Will display the subpages of a chosen Parent page (as of version 1.1 will show subpages of current page if required).</li>
<li>Customizable Title with the choice of using the Parent Page Title or writing your own</li>
<li>Compatible with subpages of subpages. You have the option of displaying Subpages of the Subpages being displayed (This is theme dependant, your theme must support submenus in the sidebar for this option to function as expected)</li>
<li>If there are no subpages to display the widget outputs nothing rather then leaving a heading without any content</li>
</ul>
This widget was built using the new WordPress Widget API and as such it supports multi instance copies. For this reason it means that this widget is only supported by WordPress 2.8 and above. If you have a requirement for this plugin for a version of WordPress lower then 2.8 please contact me on the plugins homepage via a comment.

LJ Subpages Widget

LJ Subpages Widget allows you to display a menu listing subpages from a chosen page.

Parent Page Menu vs LJ Subpages Widget