Multiping - Get MultiPing Support. Network monitoring in 60 seconds. MultiPing provides a graphical view of network performance, monitors hundreds of targets, and sends automatic alerts..
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
Fping (open source) - fping is a program to send ICMP echo probes to network hosts, similar to ping, but much better performing when pinging multiple hosts.
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.