From BlenderWiki

Smooth Modifier

Description

This modifier smooths a mesh by flattening the angles between adjacent faces in it, just like the Smooth button in the Editing context, Mesh Tools panel. So it smooths without subdividing the mesh – the number of vertices remains the same.

This modifier is not limited to smoothing, though. Its control factor can be configured outside the [0.0, 1.0] range (including negative values), which can result in interesting deformations, depending on the affected mesh.

Options

Smooth modifier.

X, Y, Z

Toggle buttons to enable/disable the modifier in the X, Y and/or Z axes directions.

Factor

The factor to control smoothing amount. The smoothing ranges from 0.0 to 1.0 (0.0: disabled, 0.5: same as the Smooth button, 1.0: maximum). Alternatively, values outside this range (above 1.0 or below 0.0) distort the mesh.

Repeat

The number of smoothing iterations, equivalent to pressing the Smooth button multiple times.

VGroup

A vertex group name, to restrict the effect to the vertices in it only. This allows for selective, realtime smoothing, by painting vertex weights.

Examples

Smooth modifier examples. Repeat option is set to 5 for all models, but each one has a different Factor value. From left to right: Top (smoothing): 1) normal Suzanne (same as Factor: 0.0); 2) Factor: 0.5; 3) 1.0; 4) 2.0. Bottom (distortion): 1) Factor: -0.3; 2) -0.6; 3) -1.0. The three models at the bottom row were subdivided twice to look nicer.