Isabel Healthcare‘s 10-year-old diagnosis decision-support system, now available as an iPhone, iPad, and iPod app, helps doctors on the go. The checklist system lets physicians enter symptoms and search a database of over 6,000 disease presentations. Information can be entered in regular text, and the results can be refined for age, gender, and travel history. Users pay a weekly ($2.99), monthly ($10.99), or annual ($119.99) subscription fee. Free trials are available here.