Name of Zaraki Kenpachi's Zanpakuto?
As of today's (16-04-2014) manga chapter (chapter 577) we got to know how Zaraki Kenpachi's Zanpakuto looks and what its name is.

The name of his Zanpakuto is Nozarashi. Also, in order to get into Shikai state the keyword seems to be "swallow".

How it looks: