Yesterday, I stumbled across AuditMyPC, which includes a number of tests (and tips) to help improve PC and network security. Most of the information is available elsewhere on the 'net but it's nice to find it all in one place. What worried me (and convinced me that the guy who runs this site knows what he is doing) is that the site was able to discover my NATted IP address. Luckily it was only on my PC with the Microsoft Java Virtual Machine (JVM) installed (unfortunately I need the JVM to use Conchango's awful time-sheeting system and, being a consultancy, time-sheeting is pretty important to us!). I know that my public IP address is easily obtained, but the ease at which a bit of Java code discovered information which should only be available from inside my firewall was a bit of a worry, especially on a fully patched Windows Server 2003 (SP1) system!
Whilst on the subject of Internet tools, some of the others that I find useful are Demon's Internet query tools as well as the tools at network-tools.com (which include a handy feature for converting base 10 addresses to dotted decimal IPv4 addresses), the subnetmask.info network tools and Gibson Research Corporation's ShieldsUp! port scanner.