I am happy to announce hostmap:
hostmap is a free, automatic, hostnames and virtual hosts discovery tool written in Python by Alessandro `jekil` Tanasi and licensed under GNU General Public License version 3 (GPLv3). It’s goal is to enumerate all hostnames and configured virtual hosts on an IP address. The primary users of hostmap are professionals performing vulnerability assessments and penetration tests.
Take a look at http://hostmap.lonerunners.net/