Wireshark

[flatrick]

Wireshark will probably be the second-most-important tool in a troubleshooters toolkit.
Being able to see exactly what's been sent and received on a network interface can often be the difference between being able to find issues with the network

[flatrick]

Tools such as SmartSniff offers similiar capabilities, but I have yet to find a more capable tool than Wireshark. But if your needs are simpler, SmartSniff from NirSoft might be easier to work with.