What is your favorite method of debugging?

Mine is a debug log. I like a key value setting for enabling/disabling, and logging most transactions, calculations, and variables, even if they seem trivial. I've been able to locate bugs much quicker with detailed logs while some coworkers are still stepping through the process line by line. I don't fault the step method as I use it when logging uncovers nothing (it usually means I didn't log something critical :p) or when logging is not possible.