Popular ones, for example, MedScape or Epocrates have huge databases of diseases, how any one of them usually presents, how it's diagnosed, labs, common treatments, differential diagnosis, etc. Quite useful when you're trying to answer some difficult question you're being asked on rounds, when you're reviewing for morning reports or stuck on a patient. I've also seen attendings use them to learn about new treatment protocols or drugs.
I also know of some like the New England Journal of Medicine app, which would play nicely with my subscription to their online service, but I've only seen it on iOS. There's an app on Android and iOS that presents all the eponyms known to man, which is quite useful when you encounter a dark name you've never heard about.
I believe there are others that let you learn to interpret an EKG, an EEG, lab work, etc. Harrison's Internal Medicine book is available on WP7, so that is much better to carry around in your white coat than a full-blown edition or even the 'baby' Harrison.
Possibilities are endless.