Stairways Software (http://www.stairways.com) has released Mounties Feedback Trainer 1.0 on the Mac App Store for US$9.99. The app was designed to help you learn to correctly identify feedback tone frequencies, a skill needed by budding sound engineers and useful for any musician.
You can use Mounties Feedback Trainer train your ear to detect the various frequencies and then test yourself to see how well you know which frequency is which. It includes includes a built-in equalizer so you can adjust the volume of the tones for the variable output of your particular speakers. Mounties Feedback Trainer requires Mac OS X 10.6.6 or greater.