The relationships between notes and frequencies.
An engineer told me many years ago that if you can recognize the note of a certain frequency then you'll be able to EQ your mixes much faster and with more ease. So if you are mixing a track and hear that there's a hump in the low end brought on by the bass playing a C and you know that this particular C is around 13oHZ (it's actually 131HZ) then you can reach for the low band EQ and cut that frequency. Most engineers will boost the low band at any frequency and start to sweep the band until they find the offending tone then they'll cut it. Don't you think it would be much more efficient to know the note and frequency through ear training so all you have to do is reach for the frequency and cut it? No more sweeping needed. Just know the note, relate that note to the frequency and make your adjustment. This was eye opening to me! Ear opening actually, but you get my point.
Below is a chart that shows the note to frequency relationships as well as the frequency range for the most popular wester musical instrument. [Click the chart to see a larger version]