Lsof. Lsof is the sysadmin/security über-tool.
I use it most for getting network connection related information from a system, but that’s just the beginning for this powerful and too-little-known application. The tool is aptly called lsof because it “lists open files“. And remember, in UNIX just about everything (including a network socket) is a file. Interestingly, lsof is also the Linux/Unix command with the most switches. Command-line Fu < The best UNIX commands on the web. OS X Sudo vs. Root: The Real Story.