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Snort - Intrucion Detection http://www.snort.org/
W3af - Web Application Attack http://w3af.org/
NetCat - networking utility http://netcat.sourceforge.net/
THC-Hydra - network logon cracker http://thc.org/thc-hydra/
Wapiti - Web application vulnerability scanner http://wapiti.sourceforge.net/
Rapid7 - http://www.rapid7.com/