There are quite a few that are helpful - it's just a matter of finding one or two that you like using and getting familiar with so they're actually good resources. Sometimes if you use too many apps, then you can get overwhelmed because you dont know which one to use in which situation, etc.
Here are a few that I've used for different things over the last few years:
Epocrates
MedCalX
NCCN Guidelines
John Hopkins ABX guide
Evernote - if you have your own quick notes or reminder lists, this can be helpful as a very quick reference. You can put in things like what a specific attending may want in their OR for a certain case or something like that.