In This Section Widget for WordPress takes any custom menu you choose, and parses it to find the current page and any pages in the same section, outputting a set of links for your sidebar.
Sections are defined by child menu items, or sub-menus. A top level menu item has pages in the same section if there is a sub-menu (i.e it has child menu items) defined for it. If there is no sub-menu a ‘No other pages in section’ message is displayed. Pages that are part of a sub-menu show the siblings of that page in the list.
Download: http://wordpress.org/extend/plugins/in-this-section/
