Epocrates is amazing, especially if you can get your school to pay for the extra clinical content. It has pathophys, treatment plans, diagnostic criteria even pathology pictures, if your school doesn't pay for it, its like $200 a year, which sucks. Medscape, skyscape and medref are almost as good for clinical content and they are free.DxSaurus is a good source for differentials. emedicine on call is really cool for treatment plans (even better for new interns). I also like netters anatomy, which is exactly what it sounds like. all of the netters flash cards on the phone. I really like imurmur, which teaches you all about the heart sounds.
I also use the usmle2 clinical content strategies app, which is also based on flash cards.
Hope this helps