I was looking for something to test my ears as I've been out of practice since I was in school. We used to have half hour classes of this, and it was the most brutal class we had, and always made us feel exhausted afterwards.
Basically, this helps you learn to identify which frequency ranges are being boosted/cut. It has modes for identifying pure sine wave frequencies, as well as identifying boosts or cuts (way harder than boosts) of various frequencies, at different difficulty levels/amounts of boost/cut. It provides a variety of different solo instruments, as well as allowing you to use your own music.
Basically, throw on a decent set of headphones, and feel really bad about your own hearing, but gradually improve it
[VIDEO]http://www.youtube.com/watch?v=mfTyxOnm4GQ]Improve Your Mixing Skills! Quiztones for iOS - YouTube[/VIDEO]
Quiztones Frequency Ear Training Apps for Audio Engineers, Producers and Musicians
Edit: also a good way to find out your headphones are shit in the extreme highs and lows
Heh, that is a cool idea, and for $5 it might be worth checking out.