Couples trying to conceive
People whose traumatic experience in high-school calculus makes them start sweating at the sight of a coordinate graph.
Seemingly borrowed from military intelligence, this puppy can sort through mountains of fertility data, from cervical fluid to body temperature to severity of premenstrual symptoms, and tell you when you're at your most fertile. It's more expensive than some similar apps on the market, but does a LOT more. Not for the faint of heart, but for couples serious about getting pregnant (or serious about avoiding it), it's worth the $10.
Buy on iTunes