Trial by Data - XDebug

Summary

A PHP extension which provides debugging and profiling capabilities.

I use this application primarily to check on code coverage. It provides a GUI front end that points out classes or sections of code that have not been sufficently tested.
As you drill down ti will show the actual lines of code color coded to indiciate whether or not it has been executed by one or more tests.
Hovering over a line will reveal a popup that lists all the tests that execute the given line.