Richard Vencu

This plugin creates and maintains one or multiple hierarchies of blogs in the WPMS network. Options screens appear in network admin dashboard. The blogs hierarchy can be used for several purposes such as:
<ol>
<li>consolidate posts (or latest posts) from descendant blogs only</li>
<li>display a list of descendant blogs or even a tree of blogs from any node</li>
</ol>
Other features
<ol>
<li>Hierarchy cannot loop (cannot set as parent one of the descendants or itself)</li>
<li>The main site (blog_id 1) cannot have a parent</li>
<li>Individual blogs may be taken out of hierarchy</li>
<li>Multiple independent hierarchies may be created 
Limitations: this plugin is not optimized for large installations. More than 50 blogs can be hard to manage.</li>
</ol>

Network Sites Hierarchy

This plugin creates and maintains one or multiple hierarchies of blogs in the WPMS network.

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.

Network Sites Hierarchy vs WenderHost Subpages Widget