Nmap - Nmap Free Security Scanner, Port Scanner, & Network Exploration Tool. Download open source software for Linux, Windows, UNIX, FreeBSD, etc.
Technitium MAC Address Changer - Technitium MAC Address Changer (TMAC) is a freeware utility to instantly change or spoof MAC Address of any network card (NIC).
Eusing Free IP Scanner - Free IP Scanner: a very fast IP scanner and port scanner.
TCP Optimizer - SpeedGuide.net - The Broadband Guide. Cable modems, DSL, Wireless, Network security. Free speed tweaks and TCP/IP tools for optimizing system performance.
Angry IP Scanner - Open-source and cross-platform network scanner designed to be fast and simple to use
Axence netTools - Axence netTools is completely free of charge, both for home and commercial use – all you have to do is to register. A tool for professionals – and others.