Brian Layman

This plugin allows the creation and display of custom sidebars for any page (or post) on your site. On each page, you can choose which of your current theme’s sidebars will be replaced. The replacement works for all descendants of a replaced page.
In this fashion you can: 
1. Easily create a CMS like site with unique sidebars for every page, even if your theme doesn’t support it. 
2. Create a sidebar with a Cart widget that displays on store pages created by the WP-E-Commerce page, but not anywhere else on the site. 
3. Use parent pages to create themed areas of your site, without creating custom page templates.
This plugin is compatible with every theme and all widgets because it hijacks your theme’s sidebars instead of replacing them.
<h3>Use</h3>
<ol>
<li>Edit a page or create and save one. It must at least have a title.</li>
<li>In the ?Custom Sidebar? section, check ?Activate a custom sidebar? and select the sidebar to replace.</li>
<li>On the Appearance->Widgets screen, place the widgets on the newly created sidebar that matches your post. </li>
</ol>
That?s it. No code. No CSS changes. No custom templates.
This video describes the usage of the plugin: 
 <iframe loading="lazy" class="youtube-player" width="640" height="360" src="https://www.youtube.com/embed/_U2XfrU7nZQ?version=3&rel=1&showsearch=0&showinfo=1&iv_load_policy=1&fs=1&hl=en-US&autohide=2&wmode=transparent" allowfullscreen="true" style="border:0;" sandbox="allow-scripts allow-same-origin allow-popups allow-presentation allow-popups-to-escape-sandbox"></iframe>

Per Page Sidebars

The Per Page Sidebars (PPS) plugin allows blog administrators to create a unique sidebar for each Page. No template editing is required.

Gregor Capuder

ProteusThemes

Primoz Cigler

<a href="https://codeship.com/projects/164619" rel="nofollow ugc"> </a>
Replace the theme default sidebars with your newly created sidebars.
This plugin was forked from the <a href="https://wordpress.org/plugins/custom-sidebars/" rel="ugc">Custom Sidebars plugin</a>, which we used for a few years, but their plugin development stalled, so we were forced to create this plugin. We really appreciate the work of developers of the original plugin!
This plugin allows you to create your own custom sidebars, configure them by adding widgets, and replace the default sidebars on posts or pages you want in just few clicks.
Here is the tl;dr list of the differences from original source:
<ul>
<li>Compatibility with the latest stable PHP version (namely PHP 7.0+).</li>
<li>Removed the “archive pages” functionality – replacing the custom sidebars on archive pages is not possible (e.g. on certain categories).</li>
<li>Open-sourced development on <a href="https://github.com/proteusthemes/custom-sidebars-by-proteusthemes" rel="nofollow ugc">GitHub</a>. Issue reports and pull requests welcome!</li>
<li>Covered with PHP unit tests.</li>
<li>Fixed couple of JavaScript errors.</li>
<li>Removed WP admin pointer.</li>
<li>Modern workflow with Grunt and Bower.</li>
<li>Removed heavy advertising.</li>
</ul>
<h3>Contact and Credits</h3>
This plugin is being developed by <a href="https://www.proteusthemes.com/" rel="nofollow ugc">ProtuesThemes</a>.
The original plugin: Custom sidebars is maintained and developed by <a href="http://premium.wpmudev.org" rel="nofollow ugc">WPMU DEV</a>. Original development completed by <a href="http://marquex.es/" rel="nofollow ugc">Javier Marquez</a>.

Custom Sidebars by ProteusThemes

Allows you to create custom sidebars. Replace sidebars for specific posts and pages.

Per Page Sidebars vs Custom Sidebars by ProteusThemes