Felix Arntz

The plugin adds two taxonomies to the WordPress media library which are then available to categorize and tag your attachments. By default, these taxonomies, although sharing the same names and behavior, are separate from the default post taxonomies, but this can easily be changed if desired.
The plugin follows WordPress Core principles and offers a lightweight alternative to similar approaches which often tend to be incredibly flexible, but at the same time complicated and bloated. And if you have a little knowledge of code, you should be able to adjust the plugin exactly to your needs if the default configuration doesn’t satisfy you.
<h4>Features</h4>
<ul>
<li>Adds categories and tags to the Media Library (independent from the regular post categories and tags)</li>
<li>Inserts filter dropdowns for attachment taxonomies into the media toolbar and media modal</li>
<li>Allows to pick taxonomy terms for attachments from within the Attachment Selection & Edit modals</li>
<li>Adds a setting for the default attachment category</li>
<li>Enhances the <code>[gallery]</code> shortcode so that images of a specific attachment taxonomy can be included automatically</li>
</ul>
<h4>Quality attributes</h4>
<ul>
<li>Lightweight plugin following WordPress Core principles</li>
<li>“Decisions, not Options”</li>
<li>Can easily be used as a must-use plugin</li>
<li>Provides a flexible API to add other attachment taxonomies or disable the existing ones for developers</li>
<li>Provides an easy-to-use <code>has_default</code> argument that can be used when registering an attachment taxonomy in order to automatically add a setting for the default taxonomy term</li>
</ul>

Attachment Taxonomies

Works perfect!

Now this is a piece of art!  No bloat, no hassle.  Well done Felix!

Piece of Art

This plugin adds categories and tags to the WordPress media library - lightweight and developer-friendly.

wlindley

Two widgets are provided, for displaying a vastly condensed 
hierarchial list either of pages, or of categories and custom 
taxonomies. Designed for large sites with hundreds of pages, or when 
using custom post types or custom hierarchical taxonomies.
With Pages: Always displays the current page, the pages above it back 
to the home page, and the current page’s children. Detects nested 
categories and custom taxonomies, and likewise displays the categories 
or taxonomies above and below the currently displayed one.
The behavior of the widget is all configurable through its 
configuration screen, as follows:
<ul>
<li>The Categories widget will also display custom taxonomies. 
The admin screen includes a list of the defined ones; 
leave the field blank to use the built-in post categories.</li>
<li>The title is optional (blank will emit no heading); with 
a blank heading and top-level entries (pages/categories/ 
taxonomies) disabled, you can have the widget only display 
when the user is “currently inside” a page/post that matches 
what the widget is set to display.</li>
<li>Pages may be sorted in Menu Order or alphabetically. 
To sort pages in menu order, with pages of the same order 
alphabetical, set the Sort Field parameter to: 
menu_order,post_title … some sites have reported issues 
and seem to require that only menu_order be specified.</li>
<li>Categories and custom taxonomies may be sorted by name, 
slug, or other fields displayed on the admin form.</li>
<li>Sort order can be ASC or DESC</li>
<li>Siblings to the current page, if enabled, means that child 
pages of the current page’s parent will be displayed. 
Disabling this makes for shorter page lists but the user 
might not be able to move “sideways” in the site.</li>
<li>If there are many top-level pages (pages with no Parent set), 
you can disable their listing.</li>
<li>If your site has an Introduction or splash page, as a static home 
page, you might want to disable showing it (presumably it is 
accessible through an icon link). However, if the homepage has 
children, it will always be displayed (otherwise you could never 
navigate to those pages).</li>
<li>For Pages, you may optionally specify a meta-key and value 
which must be matched for pages to be listed in the widget.</li>
<li>Also for Pages, you may select posts that match a given 
Author or post status. Leave these blank for “all authors” 
and “publish” status.</li>
<li>Empty categories/taxonomies may be hidden.</li>
<li>Both plugins permit specifying either an include list or 
an exclude list. These are comma-delimited lists of 
ID numbers. You may specify either include or exclude, not both.</li>
</ul>
Spanish translation provided courtesy Andrew Kurtis of WebHostingHub.com 
Thanks, Andrew!

Hierarchical Pages Widget

Two widgets are provided, for displaying a vastly condensed

Attachment Taxonomies vs Hierarchical Pages Widget