Nmap - Nmap Free Security Scanner, Port Scanner, & Network Exploration Tool. Download open source software for Linux, Windows, UNIX, FreeBSD, etc.
iperf - A TCP, UDP, and SCTP network bandwidth measurement tool
Angry IP Scanner - Open-source and cross-platform network scanner designed to be fast and simple to use
JPerf - This project gives a better UI and new functionalities to the initial JPerf 1.
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.
TamoSoft Throughput Test - TamoSoft Throughput Test is a utility for testing the performance of a wireless or wired network.