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.

DaveE

<p>The default Recent Posts widget is exceptionally basic. I always find myself in need of a way to easily display a selection of posts from any combination post type or taxonomy. Hence, Flexible Posts Widget.</p>
<p>Flexible Posts Widget (FPW) is more than just a simple alternative to the default Recent Posts widget.  With many per-instance options it is highly customizable and allows advanced users to display the resulting posts virtually any way imaginable.</p>
<h4>Features & options</h4>
<ul>
<li>Customizable widget title</li>
<li>Get posts by post type(s) and/or taxonomy & term(s) or directly by a list of post IDs.</li>
<li>Control the number of posts displayed and the number of posts to offset.</li>
<li>Option to display the post feature image.</li>
<li>Select the post feature image size to display from existing image sizes: thumbnail, medium, large, post-thumbnail or any size defined by the current theme.</li>
<li>Order posts by: date, modified date, ID, title, menu order, random, Post ID Order; and sort posts: ascending or descending.</li>
<li>Each widget’s output can be customized by user-defined templates added to the current theme folder.</li>
<li>Multi Language support. Compatible with <a href="http://wpml.org/" rel="nofollow ugc">WPML</a> and <a href="https://wordpress.org/plugins/polylang/" rel="ugc">PolyLang</a> for sure. Not tested with other multi-language plugins, but it should work.</li>
</ul>
<h4>Plugin Hooks</h4>
<p>Flexible posts widget currently has two public hooks:</p>
<ul>
<li>Filter: <a href="https://plugins.trac.wordpress.org/browser/flexible-posts-widget/trunk/includes/class-fpw-widget.php#L191" rel="nofollow ugc"><code>dpe_fpw_args</code></a> allows filtering the query vars before submitting the widget posts query.</li>
<li>Filter: <a href="https://plugins.trac.wordpress.org/browser/flexible-posts-widget/trunk/includes/class-fpw-widget.php#L354" rel="nofollow ugc"><code>dpe_fpw_template_{$template_name}</code></a> filters the template file path used to display the widget output.</li>
</ul>
<h4>Future updates & feature requests list</h4>
<ul>
<li>Use search box instead of ID text field for post id’s</li>
<li>Shortcode functionality.</li>
<li>Get posts by Author.</li>
<li>Filter out the post currently being viewed.</li>
<li>Get posts from the same archive (term/post type/etc).</li>
<li>Limit results by a time period.</li>
</ul>


Flexible Posts Widget

This has got to be the most useful plugin of its kind.



Seriousl excellent plugin

An advanced posts display widget with many options. Display posts in your sidebars any way you'd like!

Faceted Search vs Flexible Posts Widget