Simon Prosser

Debugging tool for theme authors and reviewers.
This tool intercepts all debug information and prints it all out neatly into the footer. It also checks each page for W3C validation. 
This plugin is released as a tool to aid the development of themes and plugins for WordPress and can be used to aid debugging your theme before submission to the themes directory.

Debogger

Debugging tool for theme authors and reviewers.

Dan-Lucian Stefancu

Mircea Tihu

Daniel Telbis

Dream Production

A fast/small debugger integrated into the WordPress Admin Bar, made for identifying the template used for displaying current page.
It was built for fast debugging old projects or other people’s work.
It adds some other potentially useful informations in the dropdown list:
<ul>
<li>the current theme (useful for child themes)</li>
<li>total queries on current page</li>
<li>execution time</li>
</ul>

DP Debug Menu

Very simple and handy plugin. Shows number of queries, too - very good! I like it!

Great!

Quickly shows the template used for current page, number of queries, and execution time for PHP code.

Debogger vs DP Debug Menu