Surface Constraint Tools is designed to simplify the process of modeling on the surface of another mesh. Therefore, you may find it useful for retopology tasks.
For example, the “Surface Constraint” section of the UI, as seen above, allows a user to select a target on which to constrain the vertices of the active mesh. The “Mesh Brush”, “Shrinkwrap”, and “Smooth Vertices” operators comply with this constraint. Moreover, if you would like for the constraint to affect all mesh operators, simply enable the button with the car icon.
Proper documentation does not yet exist. The addon is meant to be intuitive, but I can address any usage concerns in this thread should they arise.
Here is a link to the addon: https://github.com/fedackb/surface-constraint-tools. Click the “Download ZIP” button in the lower, right corner of the page.
To install the addon, click the “Install from File…” button within the “Addons” tab of Blender’s “User Preferences” area, and select the downloaded zip file.