For math contest enthusiasts, if you are preparing for MATHCOUNTS or other middle school and high school math contests, then the MATHCOUNTS Trainer App is a good app that you can try. The app contains thousand of problems from previous School, Chapters, and National Mathcounts. Note that all problems in this app have full solutions. For those who are not familiar with MATHCOUNTS, it is a nationwide mathematics competitions for middle school students in the United States.
In using this app, you can track your progress, compete with other players, and climb the Leaderboard.
The MATHCOUNTS Trainer app is FREE and available for iPhone, iPad, and iPod Touch. It also works offline.