The Spline Relax modifier smooths out splines by relaxing knot positions and/or handles.
Modify panel > Modifier List > Object Space Modifiers > Spline Relax Modifier
Freehand spline illustration of a ducky, before (left) and after (right) application of Spline Relax modifier.

Displays the resulting knots.
Sets the amount of relax applied with each iteration, from 0 to 1.
Sets the number of times the relax function is applied.
Applies the relax function to each knot position.
Applies the relax function to knot tangent handles.
Scales the relax function according to the knot soft selection amount. If there is no soft selection, the option is ignored.