Fyodor, the author of Nmap, has released the results of his 2006 network security tool survey. This list is full of tools that can assist in network auditing, defense and forensics. Although it is near the top of my personal list, nmap didn't make the list because Fyodor excluded it. The list includes a short description, cross links leading to categories, intuitive icons to show what OS it runs on natively and icons for availability of source code, GUI, and CLI.

You can find the list at http://SecTools.Org

From that link
`I (Fyodor) asked users from the nmap-hackers mailing list to share their favorite tools, and 3,243 people responded. This allowed me to expand the list to 100 tools, and even subdivide them into categories. Anyone in the security field would be well advised to go over the list and investigate tools they are unfamiliar with. I discovered several powerful new tools this way. I also will be pointing newbies to this site whenever they write me saying "I don't know where to start".
Respondents were allowed to list open source or commercial tools on any platform. Commercial tools are noted as such in the list below. No votes for the Nmap Security Scanner were counted because the survey was taken on a Nmap mailing list. This audience also means that the list is slightly biased toward "attack" tools rather than defensive ones.'