thewebist

A widget for displaying a list of subpage links. The list remains consistent regardless of where you are in the hierarchy.
Other features include:
<ul>
<li>Widget displays only on pages with subpages or on subpages.</li>
<li>List title is a link to the main page parent.</li>
<li>You can specify the link text for the list title, or you can completely hide it.</li>
<li>Sort subpages by Menu Order or Page Title.</li>
<li>Specify the depth of pages shown (all in hierarchy, all in flat list, 1 level, 2 levels, etc.)</li>
<li>Presentation utilizes minimal markup to allow for easy styling via your theme’s CSS.</li>
</ul>

WenderHost Subpages Widget

A widget for displaying a list of subpage links. The list remains consistent regardless of where you are in the hierarchy.

Andreas Norman

This plugin simply lists all subpages from a selected parent page (with option do exclude specific pages) in the sidebar. This widget can be placed many times in your sidebar.

SubZane Sub Page List Widget

Lists all subpages from a selected parent page with option do exclude specific pages.

WenderHost Subpages Widget vs SubZane Sub Page List Widget