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.

Matteo Enna

My Wine Repository

<p><strong>Taxonomy Meta Box Filter</strong> enhances the WordPress admin interface by adding a simple search field above each taxonomy metabox.<br />
It supports default taxonomies (like categories and tags), as well as custom taxonomies and custom post types (CPTs).</p>
<p>Perfect for sites with a large number of terms, allowing you to quickly find and select the ones you need while editing posts.</p>
<h3>Features</h3>
<ul>
<li>Search field added to every taxonomy metabox</li>
<li>Works with hierarchical (categories) and non-hierarchical (tags) taxonomies</li>
<li>Compatible with custom taxonomies and CPTs</li>
<li>Script only loads on the <code>post</code> and <code>post-new</code> admin screens</li>
<li>Fully translatable</li>
</ul>
<h3>License</h3>
<p>This plugin is licensed under the GNU General Public License v2 or later.</p>


Taxonomy Meta Box Filter – Search taxonomies from the editor

Adds a search field above each taxonomy metabox in the post editor, making it easier to manage categories, tags, and custom taxonomies.

Faceted Search vs Taxonomy Meta Box Filter – Search taxonomies from the editor