Have you ever needed to know exactly what connections are being made to and from your computer ? Or what port are open... and what processes are opening them ? Sure many of you may know how to use windows' netstat utility.. but nothing compares to the power of TCPView
www.sysinternals.com
TCPView is a GUI version of netstat on steroids. Not only does it show you all active connections to your computer (including what state they're in eg. established, time wait, fin wait etc) but it shows you which process is running them. It gives you an instant picture of whats going in and out of your machine.
Example uses :
a. You find some ports open on your machine and need to know what trojan might be using them
b. You are connected to a friend and need to find what his IP address is
c. You want to see what processes are listening for an inbound connection
d. You want to see whether a connection you established is still active or is timing out
e. You just want to know whats going on
This little utility is all of 35kb, and complete pure freeware

It goes onto my CD of essentials that I install straight after bringing a machine up. BTW the sysinternals guys make lots of other cool software.. check out the Blue Screen Of Death Screensaver which uses actual info from processes on your system to display fake blue screens as a screensaver

very trippy.. I actually rebooted the system once as I saw the error and forgot I'd installed the screensaver !
Sahir