Math
Inverse tangent. It maps a wide input range into a limited curved output, so it can be used as a gentle saturating waveshaper as well as a mathematical function.
Use moderate values first, then increase modulation depth while listening for clipping, aliasing, or excessive filter resonance.
atan(2.5*sin(X))
For pitched oscillator functions, X already contains the current MIDI note pitch and transpose information. Prefer saw(X), sin(X), or pwm(X, 0.4) unless you intentionally want an octave or ratio shift such as 0.5*X or 2*X.