CIRT.net is website providing some interesting tools to help webmasters secure their servers and their websites. Best of all it’s GPL!
Have a look at Nikto. From the website:
Nikto is an Open Source (GPL) web server scanner which performs comprehensive tests against web servers for multiple items, including over 3100 potentially dangerous files/CGIs, versions on over 625 servers, and version specific problems on over 230 servers. Scan items and plugins are frequently updated and can be automatically updated (if desired).
In todays world no server is safe, and with exploits now being discovered in more and more websites it doesn’t improve the admin’s odds.