Retopology

There are many situations where it may be advantageous to redo the toplogy of an existing model.  Perhaps you got it from a CAD software which doesn't care about topology.  Or perhaps you are remodeling something for animation.  Whatever the case, it may be best to just remodel with conventional methods (box modeling).  In other cases, it may be best to enable snapping to the mesh's face-normals as a guide. 

This entry will summarize the method that can be used to perform a retopology of an existing mesh.  In my opinion, this is best suited for complex organic shapes. 

For this example, we'll start with a Make Human head.  Note that the make-human head does has a perfectly good topology, this is just for example.

The first step is to add an mesh, in this case we are using a plane.  Give it a color so you can distinguish it form your reference model.  Now, with the plane selected, switch to edit mode [TAB], and select all the vertices [A] and delete [X] them.

In the Snap combo box, switch the Snap Element to Face, and turn on the magnet.  As well you want to enable the Project Onto Objects button (the one with the circle and dotted box on the far right).  This will cause all vertices to that are created near the surface of the head to snap to the mesh faces.

You can start modeling, by adding a vertex [CTRL + Left-Click] anywhere "on" the head.  With that vertex select, move it [G], and you'll notice that once you move it it will snap to the surface of the head.  As you begin to extrude from this vertex and create faces, you'll notice that all the faces created follow the base head model.  Note that when performing some operations such as loop-cuts [CTRL+R] that the new vertices may not snap.  A simple move [G] in place will cause it to snap.

When hiding the face, you can see how the vertices take the shape of the face even though you don't have to painstakingly adjust them to match.  

This method has many benefits in that it allows you to focus on topology rather than exact position of vertices.  Case in point, my mesh of the right eye, is quite bad, so I can use this technique to focus on improving my topology generation.

A video version of this technique can be found here:  Blender Cookie