FranH

Dummy payment gateway for developers. Simulates IPN. Always marks Order as paid.
Useful for testing and developing. Never use it on live sites!

WooFakePay

Dummy payment gateway for developers. Simulates IPN. Always marks Order as paid.

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.

WooFakePay vs Admin Bar Tools