hostmap is a free, automatic, hostnames and virtual hosts discovery tool written in Ruby 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.
The major features are:
DNS names and virtual hosts enumeration
Multiple discovery techniques, to read more see documentation.
Results correlation, aggregation and normalization
Multithreaded and event based engine
Download and more info: http://hostmap.sourceforge.net