Wireshark - Wireshark is a network protocol analyzer for Unix and Windows. It lets you capture and interactively browse the traffic running on a computer network.
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..
Ettercap - Ettercap is a suite for man in the middle attacks on LAN.
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.
tcpdump - tcpdump is a common packet analyzer that runs under the command line.
Ping Meter Gadget - Customizable, graphical, semi-transparent ping (latency) meter.