Andrea Porotti

Using this plugin you can attach notes to some elements in the WordPress dashboard, such as:
<ul>
<li>posts</li>
<li>pages</li>
<li>custom post types from other plugins (except the notes from this plugin)</li>
<li>users</li>
<li>plugins</li>
<li>dashboard sections</li>
<li>the whole dashboard</li>
</ul>
It can be helpfull if you manage a site with other people or just to take notes for yourself.
Features
<ul>
<li>manage notes like the standard posts by opening the Notes page from the admin menu.</li>
<li>while editing a note, use the meta box on the right to set note properties (position, color,…)</li>
<li>manage section and global notes from the admin bar.</li>
<li>manage item notes (eg. posts and pages) from the custom column in the admin pages.</li>
<li>get notes details by hovering the mouse on the “Info” links or clicking on the “I” icons.</li>
<li>add text, images and links in the note content.</li>
<li>deleted notes go to the trash, so they can be restored.</li>
<li>automatically discovers custom post types added by other plugins (eg. events, books…).</li>
<li>creates a list of the dashboard sections based on the admin menu items.</li>
</ul>
Configuration
Settings for the plugin are available on the Settings -> JAMP Notes page.
Please note:
<ul>
<li>by default the plugin data is kept after uninstall. You can choose to delete notes and settings enabling the data removal option.</li>
<li>after activation, the plugin enables notes for all the existing public post types. If you then install other plugins which create new post types, you have to manually enable them in the JAMP settings.</li>
</ul>
Permissions
The notes are currently available only for the users with the “Administrator” role. 
Each Administrator can manage all notes.
Support
If you find any errors or compatibility issues with other plugins, please let me know in the support forum. Thanks!
Privacy
This plugin does not collect or store any user data. It does not set any cookies and it does not connect to any third-party services.

JAMP Notes (Just Another Memo Plugin)

This plugin allows you to attach notes to some WordPress elements like posts, pages, dashboard sections and more.

jamiebergen

Have you ever returned to a site that you built a while back and asked, “Why did I install this plugin?” This plugin provides an extra column on the Plugins page that enables you to add, edit, or delete notes about the plugins you have installed on a particular site. These notes are intended to provide documentation regarding why a particular plugin was installed and how or where it’s being used.
Features
<ul>
<li>Add as many or as few notes as you need for each plugin.</li>
<li>Edit or delete notes as desired.</li>
<li>Select an icon to go with each note to quickly convey what type of content it contains (e.g., info, warning, link, etc.)</li>
<li>Format notes using basic HTML tags if desired.</li>
<li>Any links included in the note will be automatically converted to <code>target="_blank"</code></li>
<li>Notes are added and updated via Ajax, avoiding slow page reloads.</li>
<li>Notes also display on the WordPress Updates page for any plugins that need to be updated.</li>
<li>A filter is provided if you would like to display notes beneath the plugin description instead of in a separate column.</li>
<li>A filter is available to selectively hide or display plugin notes in the admin.</li>
</ul>

Plugin Notes Plus

Very useful plugin to note why a certain plugin was installed on the site and any additional context around it, for other admins. Helps in housekeeping especially when there are more than one admins that try different plugins and forget to remove anything that is not wanted.

Super useful plugin for multi-admin sites

Does one thing and does it well.

I was about to build something like this and was thrilled this plugin not only existed, but is very well done! Thank you!

Exactly What I Needed!

Adds a column to the Plugins page where you can add, edit, or delete notes about a plugin.

JAMP Notes (Just Another Memo Plugin) vs Plugin Notes Plus