Black Bar is an unobtrusive Debug Bar for WordPress developers. It collects and displays errors, executed SQL queries, slow actions and hooks, theme templates, global variables, and provides a profiler.
How it helps you with development:
Debug your code with the Console
Inspect global variables (COOKIE, GET, POST, SERVER, SESSION, WP_Screen)
Measure performance of your code with the Profiler
View any PHP errors that occur when loading a page in the Console
View executed MySQL queries along with execution time and backtrace
View template files of the active theme in loaded order
View the 50 slowest action and filter hooks along with callbacks ordered by priority