Redwoodcity

WP Notepad puts a notepad textarea on your site. Visitors can jot notes without logging in, and notes are kept in their own browser’s LocalStorage — by default nothing is sent to or stored on the server.
Add a notepad in three ways:
<ul>
<li>Classic widget — add the “Notepad” widget to any widget area.</li>
<li>Block — insert the “WP Notepad” block in the editor.</li>
<li>Shortcode — place <code>[wpnotepad]</code> in post content or theme templates.</li>
</ul>
Features:
<ul>
<li>Notes auto-save to the browser as you type, and are restored on return.</li>
<li>Live character counter and a confirm-protected Clear button.</li>
<li>The textarea grows automatically to fit its content.</li>
<li>Copy to clipboard, download as a .txt file, or import a note from a file.</li>
<li>Multiple notes in one notepad via tabs.</li>
<li>Optional auto-delete (TTL): notes expire after a configurable number of days.</li>
<li>Optional, opt-in cross-device sync for logged-in users (see below). Disabled by default.</li>
<li>Accessibility: labelled textarea, character-count association, polite save announcements and visible focus styles.</li>
<li>Block styling via the editor’s color, typography, spacing and alignment controls.</li>
<li>Available in English and Japanese.</li>
</ul>
Privacy / cross-device sync: By default notes live only in the visitor’s browser and are never stored on the server. An administrator may optionally enable “cross-device sync” under Settings → WP Notepad. When enabled, only logged-in users’ notes are saved to their own WordPress user account (user meta) and synced across their devices via the REST API. Guests (logged-out visitors) always remain LocalStorage-only.

WP Notepad

Add a notepad via a widget, block or shortcode. Notes save to the browser (LocalStorage) — no login required — with optional opt-in cross-device sync.

wycks

Like a built in notepad, leave notes for clients or yourself in the WordPress admin. Attach notes to above or below the editor of a post (or page or custom post type) and also has a default “Notes” section available from the admin menu which you can hide or show.
The style of the notes section has been updated with the changes in WordPress 3.8+. Please note that the UI will not look great on WordPress version lower then 3.8 and there are no plans to fix this.
Features :
<ul>
<li>Uses WordPress’s built in custom post types</li>
<li>Functions with default WYSIWYG editor</li>
<li>Uses WordPress’s meta fields if set to display on post edit screens</li>
<li>Shows “Notes” on one page in tab format (can be excluded)</li>
<li>Will not show in front end or search</li>
<li>Requires default post privileges</li>
</ul>
Notes :
<ul>
<li>It is currently only possible to set one note per post and one location (above or below editor)</li>
<li>Works with password protection in default section</li>
</ul>
Please report issues to: <a href="https://github.com/wycks/Simple-Admin-Notes" rel="nofollow ugc">https://github.com/wycks/Simple-Admin-Notes</a>

Simple Admin Notes

Adds a simple "Notes" section to the admin menu or posts

WP Notepad vs Simple Admin Notes