Gumroad – Volume Preserving Smoothing for Blender 2.79 + 2.8

For blender 2.8 - use VPSmoothing version 2.0 and above

For blender 2.79 use VPSmoothing 1.4

Addon for Blender for smoothing mesh surfaces without too much of volume loss.

Volume Preserving Smoothing takes also into account sharp edges - they will be treated as rails for verts to slide on. This can help to maintain main shapes of smoothed object.

To access the smoothing menu use W key in mesh edit mode -> Mesh Smoothing:

Inflate smoothing
Laplacian HC smoothing
Volume smoothing
Additional parameters are available after executing operator with F6 hotkey.

Since 2.3 update here is now smoothing brush Tool located at the bottom of left Toolbar (N).

To make edge into rail edge - just mark edge loop as sharp. Now verts belonging to this edge, will move only along edge.
Features:

smooth mesh while maintaining main object shape
border mesh vertices will slide along border edges, without shrinking (or they can be frozen)
center vertices wont move away from symmetry X axis line
works with shapekeys
ability to define sliding rail edges (marked as sharp)
three edge tension methods (Uniform, Proportional to edge length, Inverse to edge length)
optimized with NUMPY