It depends on the program, but many fields will have time to moonlight at some point. In internal medicine, for example, you will have clinic months or electives with weekends off. You can easily make $500-1000 per night, sometimes sleeping during most of the shift ("Sleeping-for-dollars", "Doc-in-a-box"). Even if a program does not have moonlighting available officially, there is still usually opportunity to moonlight outside the program, and most program directors will turn a blind eye as long as it does not affect your work in the residency (it does not count as part of your 80hr work week maximum). Most places require the Step 3 be taken with some real clinic work under your belt before you can moonlight.