by Greg Winiarski on WordPress.org
BlackBox is a plugin for plugin and theme developers. It collects and displays useful debug information (errors, executed queries, globals, profiler).

Glabal variables foramted using var_export() function, syntax coloring using awesome highlight.js library (75% zoom)
BlackBox is symfony like unobstrusive debug bar attached to the top of the browser window.
How can it help you with development:
1. instantly inspect global variables (GET, POST, COOKIE, SERVER)
2. debug both frontend and admin area
3. executed MySQL queries and time it took to execute each query (useful for finding slow queries)
4. profiler for measuring performance of your plugins and themes
5. errors occurred when loading WordPress page