Eugen Bobrowski

This is tool that help developers and administrators easier getting information.
The debug bar is available on front side for everyone if WP_DEBUG is true. If you are admin you can open bar on any page. Hover the mouse cursor to bottom right corner and click on
<h4>Features:</h4>
<ul>
<li>Implementation time counter</li>
<li>Queries counter</li>
<li>Queries list (sortable)</li>
<li>Memory counter</li>
<li>Segment checker</li>
<li>Cron jobs information</li>
<li>WP_Cache counter</li>
<li>phpinfo() window</li>
<li>References menu</li>
<li>Image Puller that provide pull missed images from production to develop site</li>
</ul>
Implementation time counter
It count the time from plugin loaded action to debug bar. 
It does not consider processes that implement before and during the plugins are loading.
Queries counter
It count all what is queried via $wpdb object.
Queries list
It show all what is queried via $wpdb object.
The <code>SAVEQUERIES</code> constant must be defined as <code>true</code>.
Memory counter
Returns the amount of memory, in bytes, that’s currently being allocated to your PHP script.
Segment checker
Also you can check any segment of your code.
<pre><code>do_action('check_segment', 'segment_1');

//do somethisng

do_action('check_segment', 'segment_1');
</code></pre>
The code below returns in debug bar new item like this:
<blockquote>
segment_1: 14.14/3/1
</blockquote>
The digits there ara time, queries and how many times this code was implemented.
References menu
The default one item of this menu is Errors. It shows notices and warnings including case when <code>WP_DEBUG</code> is false.
And also you can easy add your item to this menu and print there any var_dump or anything else.
<pre><code>add_filter('wp_debug_refs', 'my_debug_tool_ref');

function my_debug_tool_ref ($refs) {

 global $post;

 $refs['my_ref_id'] = array(
 'title' => 'My ref',
 'content' => 'My ref' . var_export($post, true),
 );
 return $refs;
}
</code></pre>

Debug Tool

Show helpful debug bar at front and admin side. Tool for developers.

tamlyn

As a theme developer, you want to have WP_DEBUG enabled but you’re not interested in the deprecated warnings created by other plugins. Use this plugin to disable these messages while still displaying other errors, warnings and notices.
<a href="http://outlandishideas.co.uk/blog/2012/06/hide-deprecated-errors-in-wordpress/" rel="nofollow ugc">Plugin by Outlandish Ideas</a>

Disable Deprecated Warnings

Prevents plugins from showing deprecated errors in the WordPress admin.

Debug Tool vs Disable Deprecated Warnings