eggplantstudios

EPS Term/Tag Cloud Search Creates a widget that will present a “Tag Cloud” of terms, and allow you to pick one or more terms to search by – Giving you the opportunity to search for all posts that are tagged “Red” and “Blue”.
Features
<ul>
<li>Supports any publicly available post type</li>
<li>Supports any publicly available taxonomy</li>
</ul>
Created by Shawn Wernig <a href="http://www.eggplantstudios.ca/" title="Eggplant Studios" rel="nofollow ugc">Eggplant Studios</a>
This release is compatible with all WordPress versions since 3.0.1 (Tested. Plugin may not work as intended for older versions of WordPress).
This plugin is English only; though it is somewhat language independent.

Term/Tag Cloud Search

Creates a widget that will present a "Tag Cloud" of terms, and allow you to pick one or more terms to search by.

AndrewUlrich

This plugin creates two widgets:
<ol>
<li>A widget where you can choose from all the categories in your blog.</li>
</ol>
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.
<ol>
<li>A widget where you can choose from all the tags in your blog</li>
</ol>
You can select the tags you want to search for, and it will only show posts that have all of the tags you selected.
<h3>Known Issues</h3>
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.
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.

Faceted Search

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

Term/Tag Cloud Search vs Faceted Search