Bsurfaces is an amazing modeling and retopology add-on for Blender. It previously sold for $39 with the promise that it would be released as GPL later on. That day is now!
For those who were waiting for Bsurfaces v1.5, I'm glad to announce that it has been released :) . It's meant to work well with Blender 2.63 (official). It can be downloaded and there is a quick help guide describing the different features at www.bsurfaces.info .