#1 Health app today is the doctor's website.
Surprising that over 50% docs don't have a site, and ones who do, don't work on mobile.
So let me be the first to declare:
The doctor's website is going to be the most popular health application in the years to come.
Further (inferred) validation in this just released Pew Research study on mobile health (#mHealth).
http://www.pewinternet.org/Reports/2012/Mobile-Health.aspx
Overview
Fully 85% of U.S. adults own a cell phone. Half own smartphones, which expands their mobile internet access and enables mobile software applications.
One in three cell phone owners (31%) have used their phone to look for health information. In a comparable, national survey conducted two years ago, 17% of cell phone owners had used their phones to look for health advice.
Smartphone owners lead this activity: 52% gather health information on their phones, compared with 6% of non-smartphone owners. Cell phone owners who are Latino, African American, between the ages of 18-49, or hold a college degree are also more likely to gather health information this way."