PingPlotter - PingPlotter is a troubleshooting tool that helps IT professionals and neophytes identify, diagnose, and resolve network issues.
Nmap - Nmap Free Security Scanner, Port Scanner, & Network Exploration Tool. Download open source software for Linux, Windows, UNIX, FreeBSD, etc.
SmokePing - Smokeping is a tool for measuring latency.
Angry IP Scanner - Open-source and cross-platform network scanner designed to be fast and simple to use
WinMTR - Free Network Diagnostic Tool. WinMTR is an equivalent of mtr for Windows.
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.