Bass Mekanik is a legendary car audio producer. His career started in Byron Lee's studio in Kingston, Jamaica. Then he went to Miami and discovered the 808 boom. "All things have a resonant frequency. For example, the resonant frequency of planet earth is like 7.83 hertz." Check out this interview.
If you have a small studio, or if you’re still learning to trust your ears when listening to a mix, visual aides are really helpful. The frequency analyzer is a budget producer's best friend. The phase scope is also quite useful. But why stop there? Lissajous curves are big fun! Here's a roundup of basic spectrum analyzers and some specialties.
Not your average frequency shifter. This is a one off, custom built, nothing but tubes inside, and it sounds great. You may have heard about Mr. Sala's Mixtur-Trautonium, which is a really nice vintage synth, but his frequency shifter is just as interesting.
Here's a little trick for hardware synth owners who want to try out xenharmonics. Even if your synth doesn't have proper tuning capabilities, you might be able to get some EDO's (equal divisions of the octave, or equal temperaments) out of it. This trick works if your synth has an option called pitch slope or pitch keyfollow.