netstat (network statistics) is a command-line tool that displays network connections (both incoming and outgoing), routing tables, and a number of network interface statistics. It is available on Unix, Unix-like, and Windows NT-based operating systems.

Microsoft Windows XP - Netstat, documentation for the netstat.exe command-line program
The netstat Command (Linux), a guide to using the netstat command in Linux.
Security Now #49 - The NETSTAT Command, podcast guide to netstat from Security Now!.