I just ran across this cool little application that helps to train your ears (You mind really) to recognize frequencies.
If you are a sound engineer, like myself, then feedback is always an enemy in live situations.
Here is a "Simple Feedback Trainer". It basically acts like an equalizer in which you hear a tone, and 'guess' the frequency of it, and then pull down the corresponding fader.
Simple Feedback Trainer
What good is it? Well you can play with it for a few minutes each day, and train your ears to hear freqs, then associate those with the proper freqs as laid out on the standard EQ. By doing this, when you are ringing out a monitor system, or FOH (Front Of House) you can do it much quicker and efficiently, thus cutting way down on the wear and tear and patience of everyone else associated with the show or building.
Screen shot of the Simple Feedback Trainer.
This will also help you in hearing, catching, and being able to tune a variety of sounds being made on stage, and adjusting individual instruments channels to taste.