WebMD is a handy app that gives you some general medical advice.

There is a detailed graphics-based symptom checker, info on just about every over-the-counter and prescription drug, and advice on how to administer first aid.

WebMD makes it clear that it's not an alternative to a doctor - you should always consult one in an emergency - but it provides a helpful starting point that might save you some cash by avoiding an emergency room visit.