Roy Tanck

<h4>Redirect all email sent by WordPress to the admin’s email address.</h4>
When activated, this plugin will send all email sent by WordPress to the admin email address instead of the intended recipient.
It uses the wp_mail filter to modify the “to” address for all email sent by WordPress. When active, all email sent through WordPress’s wp_mail() function is rerouted to the admin email address as configured under “Settings – General”.
One of its uses is to allow the creation of new users during testing. WordPress requires user email addresses to be unique, and sends confirmation messages to these addresses. Using this plugin, you can use fake addresses, and still receive the activation links.

Mail Caesar!

I love plugins which you install and just use without the hassle to go through a bunch of confusing options and settings. This plugin is such a lean and clean little helper, eg. if you need to test functionality for a new website and don't want the mail to be sent to your client.

Bonus for the superb plugin name! 👍🏻

Cool stuff

Redirect all email sent by WordPress to the admin's email address.

Grégory Viguier

The plugin adds a new tab in your admin bar with simple but useful indications and tools.
<ul>
<li>Displays the number of queries in your page and the amount of time to generate the page.</li>
<li>Displays the php memory usage and php memory limits (constants <code>WP_MEMORY_LIMIT</code> and <code>WP_MAX_MEMORY_LIMIT</code>).</li>
<li>displays the php version and WP version.</li>
<li>Displays <code>WP_DEBUG</code>, <code>SCRIPT_DEBUG</code>, <code>WP_DEBUG_LOG</code>, <code>WP_DEBUG_DISPLAY</code>, and error reporting values.</li>
</ul>
In your site front-end:
<ul>
<li>Lists the template and all template parts used in the current page (template parts added with <code>get_template_part()</code>). Compatible with WooCommerce’s templates.</li>
<li><code>$wp_query</code>: this will open a lightbox displaying the content of <code>$wp_query</code>. Click the lightbox title to reload the value, click outside the lightbox to close it.</li>
</ul>
In your site administration:
<ul>
<li>Admin hooks: lists some oftenly used hooks (like <code>admin_init</code>). The indicator to the right of the line tells you how many times the hook has been triggered by a callback. A “P” means the hook has a parameter: hover it for more details. Click a hook (on its text) to auto-select its code, for example: click admin_init to select <code>add_action( 'admin_init', '' );</code>.</li>
<li><code>$current_screen</code>: displays the value of 4 properties of this object: <code>id</code>, <code>base</code>, <code>parent_base</code>, <code>parent_file</code>.</li>
<li><code>$...now</code>: displays the value of the well-known variables <code>$pagenow</code>, <code>$typenow</code>, and <code>$taxnow</code>.</li>
<li>On a user profile page, <code>$userdata</code>: : this will open a lightbox displaying the user’s data.</li>
</ul>
You can decide who’s gonna use this plugin (go to your profile page for all the settings). This way, the plugin’s items won’t show up to other users (your client for example). 
Also, a new menu item <code>Code Tester</code> will appear. There you are able to do some tests with your code.

Admin Bar Tools

I'm using this wonderfull tool in all my project and ask my studients to use it when they dev for WordPress.

Make a great job

In all my dev sites

This is one of those plugins I use on every WordPress project. The UI/UX is excellent, it's always there in the admin bar, you don't need to load anything. 

It's just a pity it's not updated to the latest WordPress versions. I sure hope this is not abandoned.

Thanks for sharing your work, @greglone.

So useful. Too bad it's not updated

Adds some small development tools to the admin bar.

Mail Caesar! vs Admin Bar Tools