The curve loop script turns an edge loop into a smooth curve.
Take a look at the video below for some examples.

Example video

Instructions on using the script

(also included in the script itself)

Select one or more vertices on the same edge loop.
These vertices along with the 2 vertices at the end of the edge loop (if there are any)
will be used as control points through which a smooth curve will be calculated.
This curve is generated using a natural cubic spline algorithm.
All other vertices on the edge loop (that is, the non-selected vertices) will be moved,
so they are located on this smooth curve.

Special cases:
- Only one vertex is selected
    If the vertex is part of several edge loops the script will prompt the user to select
    an additional vertex that is located on the correct edge loop, so the script can calculate
    a correct curve. This additional vertex (or vertices) will not be used as control point.
- All vertices in an edge loop are selected
    In this case the script operates on all edge loops that are perpendicular to the selected one.

There also is a support thread for this script at BlenderArtists.

Download

The script is available as a simple <python file> which can be directly run inside Blender's text editor
or integrated in the scripts menu (it's located in the 'Mesh' submenu). For more information on
using scripts check out the page: how to run a python script?