CG Cookie Polystrips Retopology Tool
Polystrips has been combined with Contours and is now offered as a part of RetopoFlow: https://cgcookiemarkets.com/all-products/retopoflow/
This version is no longer maintained.
The Polystrips Retopology tool is an addon for Blender that provides quick and easy ways create the key face loops needed to retopologize a complex model. Use cases include complex forms, such as human faces, creatures, and other organic & hard-surface objects.
The tool works by hand-drawing strokes on to the high-resolution object. The strokes are instantly converted into spline-based strips of polygons, which can be used to quickly map out the key topology flow. Clean mesh previews are generated on the fly, showing you the exact mesh that’ll be created.
The add-on is available for purchase from the Blender Market: http://cgcookiemarkets.com/blender/all-products/polystrips-retopology-tool/