Previously available for 40$, Bsurfaces is now available freely under the GPL license. This wonderful modeling/retopology tool should be added to your tool chest as soon as possible. For more details on what BSurfaces offers, check here.
The following guide shows how to install BSurfaces, or for that matter, any Blender plugin.
To install it, head over to this site and download it.
Open the zip file, the file you are interested in is bsurfaces.py, extract it somewhere, I will use my desktop.
Now fire up Blender. In select the menu File-> User Preferences… like such:
In the resulting dialog, select Addons, then click Install Addons…
Navigate to the location you extracted pysurfaces.py, then click Install Addon…
Now click the checkboxes to enable the plugin
And BSurfaces is now installed.