CG Cookie Contours Retopology Tool
Contours has been combined with Polystrips and is now offered as a part of RetopoFlow: https://cgcookiemarkets.com/all-products/retopoflow/
This version is no longer maintained.
The Contours Retopology tool is an addon for Blender that provides quick and easy ways retopologize cylindrical forms. Use cases include organic forms, such as arms, legs, tentacles, tails, horns, etc.
The tool works by drawing strokes perpendicular to the form to define the contour of the shape. Immediately upon drawing the first stroke, a preview mesh is generated, showing you exactly what you’ll get. You can draw as many strokes as you like, in any order, from any direction.
The add-on is available for purchase from the Blender Market: http://cgcookiemarkets.com/blender/all-products/contours-retopology-tool/