My first iOS App was released earlier this year. For some reason I didn't post about it on here. So here it is, a brief introduction.
It's pretty simple - listen to a pure tone and try to identify the frequency. Start with tones at octave spacing. When you feel comfortable move up to third octave spacing. If you want more of a challenge, try listening to filtered noise and attempt to identify the center frequency.
It will make you better at identifying frequencies in the sounds you hear. It may improve your mixes, or make you better at identifying problems in your audio.
It's all beautifully presented in native iOS 7 style. All audio is generated in real time, meaning the app doesn't need tens or hundreds of megabytes of audio files. It's only 120 KB. You won't even notice it's there. And best of all, it's free.