Cool Tool: Nessus, for good or ill
Many open source tools are powerful to the point of being dangerous, but if there was a competition for potential destruction embodied in a single application, Nessus would definitely be a contender.
Nessus is a hard-core vulnerability tester, which takes an IP address in at the one end and spews every possible security hole out the other. For systems administrators, this is a seriously powerful tool. As anyone who has seen the logs of a live Internet server will know, hackers are hitting your server all the time. While a good night's sleep is never easy for a sys-admin, securing all the problems that Nessus discovers will you slumber sweetly.
But, like the Force, such a tool can be used for good or evil.