Baseball Prospectus is one of the more revered texts among fans of sabermetrics and fantasy baseball. It's not just a compendium of every stat—for every player—you can think of. It's also the home of the essential PECOTA projection system, a generally reliable way to foresee who's going to have a monster year and who's due for a decline.
There's really no better way to get a leg up on your fantasy baseball league. And while $15 might sound hefty for an app, it's just slightly more than you'd pay for the 2011 BP paperback version at Amazon right now. Considering how much more navigable it is than that 600 page beast, it's really a no-brainer. Also available in NL and AL-only varieties at $10 a pop, if you feel that strongly about the designated hitter rule. [iTunes]