Some may find this a bit strange, but I love Prognosis - Your Diagnosis, by Medical JoyWorks ! Although it is called "One of the five best apps for Doctors", it won the prize for the Best Health App at the World Summit Awards for Mobile Apps, 2012 and has millions of downloads.
I've been using it for at least a year, and amazingly find that I have not killed too many patients although I have absolutely no medial training. The app presents you with a patient, allows you to examine the patient and select some tests, and then choose the appropriate treatments. After you discover how well you did, it provides a thorough differential diagnosis writeup that is very educational. I also enjoy reading the comments by other users of the app, mostly doctors or medical students, who may agree or differ with the treatments prescribed.
It's just a blast, to pick off a case or two while waiting for an appointment or before bed. Try it out!