DC blocker
The smallest useful filter in audio DSP: a zero at 0 Hz and a pole right next to it, removing DC and leaving everything else alone.
It fits on one line: y[n] = x[n] − x[n−1] + R·y[n−1]. The difference kills anything constant, and feedback with R just under 1, typically around 0.995, confines the effect to the very lowest frequencies. At 44.1 kHz the cutoff lands at a few hertz. Julius O. Smith at Stanford's CCRMA documented the form in his textbooks, and it has been pasted into roughly every synth since.
It belongs after anything that can create DC offset: asymmetric waveshaping, rectification, feedback loops in physical modelling. Without one, offset wanders around a loop until it eats all the headroom. The cost is a little phase shift in the bass, which nobody has ever complained about.