We talk about a lot of different types of apps here at Android Police, but very seldom do we get to discuss an app that may actually save someone's life. WebMD for Android could be that app, and while I realize that may be a stretch, the possibility is still there.
This app really is an on-the-go doctor, as it offers a ton of extremely useful features. The symptom checker is a good starting point - it brings up a virtual body, where you tap the area that is troubling you, choose your symptoms, and then read about potential causes. Once you think you've nailed the issue down, you can read medically reviewed information about the condition and discover treatment plans (including side-effects of any medication involved in said treatment).
Have a pill but you're not sure what it could be? The WebMD also offers a pill identification tool that uses pill shape, color, and imprint to decide exactly what it is.
In my opinion, one of the most beneficial features of this app is the first aid essentials section. Covering everything from insect stings to broken bones, you'll know exactly what to do should an emergency situation arise.
There are a veritable gaggle of other features as well, like voice search, maps to local hospitals and pharmacies, and an option to email the info directly from the app.
WebMD is free in the Android Market and is probably a must have app for app for anyone that breathes, which should cover basically everyone.