The Profiling Guy
I used to be on a team that maintained a complicated web application that was leaking huge amounts of memory. Even when set to 2 gigabytes, the heap would run low on space, which slowed performance to a crawl as the garbage collector furiously ate up CPU cycles. Eventually an OutOfMemoryError would occur, which slowed performance all the way to a stop.
To diagnose the problem I ended up using JFluid, which at the time was an experimental profiling technology developed by Misha Dmitriev. I was impressed enough with JFluid that I wrote an article about it for Java Developer