I am heading into my third year of medical school and have been trying to diversify my apps before hitting the wards (so I haven't tried any of these in a clinical setting yet). My findings on Android thus far:
Free:
Epocrates (which is free, but in beta and only available on android 1.6 or higher)
Skyscape: this has drugs, medical calculators for about everything and a fairly decent database on disease (its no Harrison's or Robbins, but its pretty good for a quick reference).
PubMed: abstracts for primary lit that link to the pub med site with access dependent upon what access you have
Medical Spanish: will actually state common questions and statements aloud with written spanish that can be read
Relief Center
Advanced Eye Charts
CPR
Medicinal herbs (not much detail at all, but may give you a start to go off if a patient is rather vague).
and I also grabbed a scientific calculator
Low price (which I haven't purchased yet, but intrigued me):
Diagnosaurus ($1) (i have used the online version of this and it is good for expanding a ddx)
EKG droid
progress Notes
Then there are all sort of subscriptions with a high price tag to various references and books that can be purchased: classics like the Red Book, Washington Manual, 5 Minute Consult, Harrison's, etc. I am waiting on these until I know what I will need once I hit the wards.