Kyle Charlton

Save page and post notes in the editor or while logged in, add notes from the front end using the integrated notes button found in the admin bar.
<h4>Features</h4>
<ul>
<li>Make page and post notes from both the front or back end (while logged in)</li>
<li>Front end: Lock the note box open or closed on a per page basis so you can select which pages or posts show your notes automatically</li>
<li>Front end: Movable note boxes</li>
<li>Front end: Notes are saved using Ajax which means no refresh when you click save</li>
<li>Front end: The positon of the note box saves automatically once you have finished moving it to your desired location (per page basis). </li>
<li>Front end: The note box is resizable, the dimentions are saved after each resize</li>
<li>Front end: Save status appears after every save action so you know when your notes, size and position are being saved </li>
<li>Dashboard: Dashboard widget that will display all page/post notes with links to each page</li>
</ul>
<h4>Future Updates</h4>
<ul>
<li>Record user who made note (Pro version)</li>
<li>Create multiple notes/annotations on a single page for logged in users by role (Pro version)</li>
<li>Assign users to notes (users receive notification of new notes) (Pro version)</li>
<li>Make comments on notes (Pro version)</li>
</ul>

Site Notes

very useful

A plugin that adds a note box to  your posts and pages which can be viewed in the admin bar

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.

Site Notes vs JAMP Notes (Just Another Memo Plugin)