Any half decent sysadmin will routinely test the security of his or her own systems. A good, and sensible, sysadmin will follow up those tests with an independent security audit by a professional company – preferably one which is a member of a recognised industry body (such as CREST). Finding the holes in your security mechanisms (and there will be some – probably more than you will be happy about) before the bad guys do is essential if you want to sleep at night (and keep your job).
There are a huge number of security testing tools available for free if you know where to look. Most sysadmins keep a toolbox of their favourites (nmap, nessus, ettercap, dsniff et al.) to hand ready for testing any new build. But it can sometimes be difficult to know just which tool to use, and where to get it. Enter backtack. I first came across this collection of tools as recently as february 2006 and found it an excellent resource. Essentially backtrack is a collection of all the security testing tools you are likely to need packaged into one linux distribution. Think of it as a knoppix for security testing. A complete list of all the tools in the collection can be seen here.
Bactktrack Version 3 has just hit the streets. Get it here.
(Oh, and don’t think that using a toolset like this makes you a pen-tester. It doesn’t. What it might do is make you more security aware, and a better sysadmin.)