AndrewUlrich

<p>This plugin creates two widgets:</p>
<ol>
<li>A widget where you can choose from all the categories in your blog.</li>
</ol>
<p>This is similar to what newegg.com uses, where you can select multiple categories, and it will only show posts belonging to all the categories you select, and such categories can be narrowed down by selecting their subcategories.</p>
<ol>
<li>A widget where you can choose from all the tags in your blog</li>
</ol>
<p>You can select the tags you want to search for, and it will only show posts that have all of the tags you selected.</p>
<h3>Known Issues</h3>
<p>After upgrading from 2.1 to 2.2 it is highly recommended that you deactivate the plugin and then re-activate it.  Unfortunately WordPress 3.1 no longer calls register_activation_hook when the plugins are upgraded, so this may be necessary, otherwise you may not see all your categories.</p>
<p>When using version 3.2 or above, and excluding categories, the display order may or may not be mixed up when you exclude a parent category.</p>


Faceted Search

This plugin will create widgets that you can use to conduct faceted searches using the categories or tags on your site.

Felix Arntz

<p>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.</p>
<p>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.</p>
<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.

Faceted Search vs Attachment Taxonomies