Michael Torbert

This plugin is great for someone needing to debug their own or another person’s WordPress site. It will collect and display and/or send debug information to an email address provided by the user. 
This is helpful for plugin or theme authors wanting to debug user’s sites.
Some features:
<ul>
<li>Displays system information</li>
<li>Emails system information</li>
</ul>
<h3>Arbitrary section</h3>
You may provide arbitrary sections, in the same format as the ones above. This may be of use for extremely complicated 
plugins where more information needs to be conveyed that doesn’t fit into the categories of “description” or 
“installation.” Arbitrary sections will be shown below the built-in sections outlined above.
<h3>A brief Markdown Example</h3>
Ordered list:
<ol>
<li>Some feature</li>
<li>Another feature</li>
<li>Something else about the plugin</li>
</ol>
Unordered list:
<ul>
<li>something</li>
<li>something else</li>
<li>third thing</li>
</ul>
Here’s a link to <a href="https://wordpress.org/" title="Your favorite software" rel="ugc">WordPress</a> and one to <a href="http://daringfireball.net/projects/markdown/syntax" title="Markdown is what the parser uses to process much of the readme file" rel="nofollow ugc">Markdown’s Syntax Documentation</a>. 
Titles are optional, naturally.
Markdown uses email style notation for blockquotes and I’ve been told:
<blockquote>
Asterisks for emphasis. Double it up for strong.
</blockquote>
<pre><code><?php code(); // goes in backticks ?>
</code></pre>

WordPress Debug

Displays and emails system information for debugging purposes for easy debugging of your own or a client/user site.

Silkalns

WP Chill

Brian Fegter

Chris Dillon

Razvan Aldea

For admins, developers, and support staff, Debug This provides a ton of information about your WordPress installation, all from the front-end admin bar.
<ul>
<li>The current WP_Query object</li>
<li>Blog info and options</li>
<li>Embed providers</li>
<li>Files in rendered HTML (CSS, images, JavaScript)</li>
<li>Filters and actions</li>
<li>Images sizes</li>
<li>Globals and constants</li>
<li>Menus</li>
<li>Object cache stats</li>
<li>Plugins, must-use plugins, and dropins</li>
<li>Rewrite rules</li>
<li>Queries</li>
<li>PHP and server information</li>
<li>Post attachments</li>
<li>Post types</li>
<li>Scripts and styles enqueued</li>
<li>Shortcodes</li>
<li>Sidebars and widgets</li>
<li>Taxonomies and terms</li>
<li>Themes</li>
<li>Users</li>
<li>WP cron schedules and jobs</li>
<li>WP debug log</li>
</ul>
Debug This helps you save time and effort when trying to figure out what’s going on. Instead of hardcoding debug snippets or writing complex unit 
tests for small functionality, you can simply bring to the surface what you need right from the admin bar.
For example, when viewing a single post, you can see:
<ul>
<li>a list of post attachments,</li>
<li>which menus and sidebars are being displayed,</li>
<li>the post type, post fields, and meta data,</li>
<li>the current query variables and query string,</li>
<li>the rewrite rules applied,</li>
<li>the author,</li>
<li>the terms,</li>
<li>and much more.</li>
</ul>
<h3>Extend</h3>
<h4>Debug This Functions</h4>
New debug modes can be created easily:
<pre><code>add_debug_extension(
 $mode,
 $menu_label,
 $description,
 $callback,
 $group = 'General'
);
</code></pre>
Example
<pre><code>add_debug_extension(
 'actions',
 __('Actions', 'debug-this'),
 __('$wp_actions contains all active registered actions', 'debug-this'),
 'foo_callback',
 'Filters And Actions'
);
function foo_callback($buffer, $template){
 global $wp_actions;
 $debug = print_r($wp_actions, true);
 return $debug;
}
</code></pre>
You can add links to the header of a debug mode page. Place this code within your debug callback function.
<pre><code>add_debug_header_link('http://urltolink', 'Link Label');
</code></pre>
Extensions can be removed as well using <code>remove_debug_extension($mode);</code>
No PRE Tags
If you don’t want your debug output to be enclosed in PRE tags, simply set the following in your extension:
<pre><code>Debug_This::$no_pre = true;
</code></pre>
Saved Queries and Execution Time
Retrieve saved queries and execution time by using the following static properties:
<ul>
<li><code>Debug_This::$execution_time</code></li>
<li><code>Debug_This::$queries</code> – SAVEQUERIES must defined as true</li>
</ul>
URL Helpers
<ul>
<li><code>Debug_This::get_current_debug_url()</code> – current URL with the debug query</li>
<li><code>Debug_This::get_escape_url()</code> – used for the debug escape link that links to original page URL</li>
</ul>
<h4>WP Actions</h4>
<ul>
<li><code>debug_this</code> – receives the $mode arg – outputs the debug code sent from the extension modes. The default action is set to priority 5. This allows you to prepend or append any output without conflict using less or greater priorities.</li>
</ul>
<h4>WP Filters</h4>
There are a few filters you can use to customize Debug This to your needs:
<ul>
<li><code>debug_this_template</code> – receives $template arg – Use your own template</li>
<li><code>debug_this_default_mode</code> – receives $mode arg – Alters the mode for the parent DT admin bar button link.</li>
<li><code>debug_this_output</code> – receives $output, $mode args – Filter debug content before it’s rendered</li>
</ul>
<h4>JavaScript</h4>
To access the built-in Debug This JS functionality, enqueue your custom script with the dependency set to <code>debug-this</code>. Your script will inherit a jQuery dependency.
Object: debugThis
<ul>
<li><code>debugThis.mode</code> – current mode</li>
<li><code>debugThis.defaultMode</code></li>
<li><code>debugThis.template</code> – current included template</li>
<li><code>debugThis.queryVar</code> – the defined query string variable</li>
</ul>
Functions:
<ul>
<li><code>isDebug()</code></li>
<li><code>getDebugMode()</code> – uses <code>isDebug()</code></li>
</ul>
Events:
A jQuery <code>debug-this</code> event is fired from the footer. You can hook into this event with the following;
<pre><code>jQuery(document).bind('debug-this', function(event, debugThis){
 console.log(debugThis);
});
</code></pre>
<h4>Helper Functions</h4>
There are three included functions to help you work with files.
<ul>
<li><code>debug_this_get_file_ownership($file)</code> – returns <code>array('name' => $name, 'group' => $group)</code></li>
<li><code>debug_this_get_file_perms($file)</code> – returns string – Example: 0775</li>
<li><code>debug_this_convert_perms_to_rwx($perms)</code> – returns string – converts permission number to RWX format – Example: 0755 folder becomes drwxr-xr-x</li>
</ul>

Debug This

Very nice, even the phpinfo() colors are displayed correctly.

Great plugin

Peek under the hood with sixty debugging reports just one click away.

WordPress Debug vs Debug This