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.

Ederson Peka

With this plugin, you can create as much “sidebars” (widget areas) as you need, and assign them in a per-page basis. (It’s only useful if your theme supports “sidebars”.)
WARNING: Updating from versions prior to 0.2.5 requires new set up.

Unlimited Page Sidebars

This plugin does exactly what it needs to and always works. It's great.

Does exactly what it needs to!

Assign one specific widget area (sidebar) to each page.

Per Page Sidebars vs Unlimited Page Sidebars