Practical insightful time domain functions
I've written a series of three blog articles about three families of time domain frequency calculation for single pure tones, real or complex.  You can find them here:


Exact Near Instantaneous Frequency Formulas Best at Peaks (Part 1)
Exact Near Instantaneous Frequency Formulas Best at Peaks (Part 2)
Exact Near Instantaneous Frequency Formulas Best at Zero Crossings


The applicability in DSP is rather limited, but if this is what you are doing, these are the formulas you want.