A simple plugin that reports the call stack trace when PHP errors occur.
A WordPress plugin that makes var_dump and print_r pretty!
BlackBox is a plugin for plugin and theme developers. It collects and displays useful debug information (errors, executed queries, globals, profiler).
Plugin for debugging. Showing the value of all PHP constants. Useful for instance for solving problems when upgrading or when changing webhoster.
Displays a list of each page's SQL queries and the functions calling them that can be searched and sorted by time, type, etc.
Adds PHP profiling support to your WordPress using Facebook's XHProf Profiler.
Log variables and view the results in your admin interface
Debug your plugins and themes.
Extremely simple mechanism for embedding FirePHP into WordPress to use FirePHP during WP Development.
A debugging toolbar for WordPress.
Prints out your webservers php settings and WordPress environmental information. Important for posting within various WordPress support forums.