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.

Nagaoka Design

This plugin helps WordPress developers by showing which template files are being used on the current page. It displays the current template file name and directory path in a convenient overlay, making it easy to identify which template is rendering the current page during development.
The plugin shows a small, unobtrusive display that can be clicked to reveal detailed information about all template files loaded for the current page.
Key Features: 
* Shows current template file name 
* Displays template directory path 
* Click to view all included template files 
* Configurable display position (top-left, top-right, bottom-left, bottom-right) 
* Customizable background and text colors 
* Admin-only display option for security 
* Debug mode option (only shows when WP_DEBUG is enabled) 
* Lightweight and performance-optimized 
* Clean, modern interface
Perfect for: 
* Theme developers 
* WordPress developers 
* Site debugging 
* Template hierarchy understanding 
* Development and staging environments
The plugin is designed to be completely safe and non-intrusive, with options to restrict visibility to administrators only.

Current Page Template Viewer

Display current template file and directory name on screen for WordPress development.

Debogger vs Current Page Template Viewer