It isn't free but it's well worth the money.
http://www.moultonlabs.com/full/product01I went to school for audio engineering and we spent 30 minutes at the beginning of one of my classes using this to train throughout the semester. It is a very effective way to train your ears. The concept, you'll hear pink noise (pretty much even volume throughout the frequency spectrum) and then they will make a 10dB boost or cut in a specific area and you have to guess where that boost or cut happened.
The boosts/cuts happen within a smaller and smaller range as you go through the disc so your ear becomes more laser focused on frequencies.
A recommendation: if you do buy them, do it at the beginning of the day and
ONLY do it for 30 min or so at a time. Any more and your ears become fatigued and the effort is wasted.
Hope this helps!