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
This plugin is available for download to be used on your WordPress self-hosted installation.