Nmap - Nmap Free Security Scanner, Port Scanner, & Network Exploration Tool. Download open source software for Linux, Windows, UNIX, FreeBSD, etc.
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
Throttled Pro - Throttled Pro optimizes your internet connection and provides QoS (Quality of Service) to Mac OS X.
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.
SpeedConnect Internet Accelerator - SpeedConnect Internet Accelerator is software for optimizing your internet connection and boost the speed as well as response.