xdebug and webgrind
Xdebug worked out of the box with webgrind in MAMP (although there was some trial and error for me because I had some non-default extensions enabled in MAMP).
Of course, it also works out of the box just fine with our development server, and as a sysadmin it gave me some good insights into why certain sites seem to hammer the heck out of certain resources. (What do you mean I have a function three includes deep that reads out an entire 10,000 record database table into an array and then never uses it again?)