The rules around moonlighting change from program to program and are different depending on what state/province you are in. I know that I'll be able to moonlight after my third year of residency (i'm doing gen surg) in both the ICU and ER. However, you have to get a special licence and get insurance to cover you. Most of the residencts I've talked to say it's financially a good deal though
I can moonlight after first year and after I have taken the USMLE step 3. As a pathology resident we can moonlight for the medical examiners office or work in the "gross" room at other hospitals. I have heard of residents doubling their incomes by moonlighting.
Yes GP, I've heard of many residents that more than doubled their salaries by moonlighting. I heard of one guy that took a year off to do research. His program didn't have enough residents to fill the call schedule, so he filled in doing call 1 in 4. He got paid well for it and made over 100,000 for the year.