TCPView - TCPView is a Windows program that will show you detailed listings of all TCP and UDP endpoints on...
Nmap - Nmap Free Security Scanner, Port Scanner, & Network Exploration Tool. Download open source software for Linux, Windows, UNIX, FreeBSD, etc.
CurrPorts - CurrPorts displays the list of all currently opened TCP/IP and UDP ports on your local computer.
Angry IP Scanner - Open-source and cross-platform network scanner designed to be fast and simple to use
TCP Optimizer - SpeedGuide.net - The Broadband Guide. Cable modems, DSL, Wireless, Network security. Free speed tweaks and TCP/IP tools for optimizing system performance.
Advanced IP Scanner - Advanced IP Scanner shows all network devices, gives you access to shared folders, and can even remotely switch computers off. Download it Free.