Vertex ‣ Smooth Vertices, Context Menu ‣ Smooth
This tool smooths the selected vertices by averaging the angles between the faces. After using the tool, options appear in the Toolbar:
The number of smoothing iterations.
Limit the effect to certain axes.
Adjusting the smooth option after using the Subdivide tool results in a more organic shape.
The Smooth Modifier, which can be limited to a Vertex Group, is a non-destructive alternative to the Smooth tool.