Janar

With this plugin you can add extra HTML or just plain text when posting a new page or post. That content will be displayed in sidebar widget. It is tested from WP 2.6 to 3.0.1.

ax-sidebar

With this plugin you can add extra HTML or just plain text when posting a new page or post. That content will be displayed in sidebar widget.

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.

ax-sidebar vs Custom Sidebars by ProteusThemes