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.
Ping Meter Gadget - Customizable, graphical, semi-transparent ping (latency) meter.
Winsock Packet Editor - WPE Pro is a packet editor.
NetTK - NetTK, Network Tool Kit, is an Open Source alternative for monitoring and troubleshooting your...
packeth - packeth is GUI and CLI packet generator tool for ethernet.
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.