Depends on the field.
For example, there are often a ton of available moonlighting opportunities in Psychiatry. That often pay really well. Fourth year psych residents also typically have a majority of their year dedicated to electives, so I personally have known people to triple their salary during PGY4 by adding a bunch of moonlighting. But no one other than a psychiatrist could pick up these opportunities.
EM has tons of opportunities.
Radiology often has moonlighting opportunities to pick up extra call/nights/whatever.
Internal medicine there's sometimes internal opportunities (specialty services, pick up extra call, something).
I've heard in rad onc there's often opportunity to be the one babysitting patients while techs do the job (some forms of radiation therapy simply require a doctor in the building who might not otherwise be there).
Etc, etc. It will vary a lot depending on field, program, and location.