i was a volunteer in the emergency room at san francisco general on and off for a few years. awesome experience, especially if you like county hospitals. there was never a night when i didn't see some type of trauma, not that that's a good thing, but you definitely learn a lot. you start out as a patient advocate, helping the staff, talking to a wide variety of patients, etc. if you log in so many hours, i believe it's 40, 50, perhaps, about one semester's worth, the volunteer center opens up more doors for you to pursue interests. one is with peds, another with their aids clinic, and another where they allow you shadow their e.r. attending docs for their entire shift. i actually believe it's a course that is offered to the ucsf ms1s and ms2s. i definitely enjoyed it a lot. walked around in the scrubs, pretty much free to experience anything, got to listen in with the group during rounds and hear the various residents present their cases to everyone, and the docs were always willing to answer any questions.
the only drawback is that you work with a wide range of doctors b/c they don't have regular shifts or anything. so if you are looking to work with just one doctor, and get a letter or something, then it's a little hard in that you have to be pretty flexible with their schedule. one day might be a 6am-2pm shift. a few days later a 10pm-6am, etc. so if you really need a letter, then it might be easier to just approach one physician.
if you need more info or what not feel free to pm me. i can try to dig up the info.
good luck