Palliative Care
Cardiology
Neurology
I didn't find radiology that helpful--unless you have really great attendings you pretty much just sit and stare over their shoulder. But it does make for a good Step 3 study month because they usually send you home early.
I also did a PM&R rotation--I wanted to see what acute inpatient rehab in a community hospital was like. It's quite different than how it is in an academic center or VA.