The value of Wireshark. I'm not sure I can overstate the importance of starting your network troubleshooting with Wireshark. On many occasions I have been witness to troubleshooters, who for all intents and purposes are experts in their respective fields, rack their brains over connectivity issues when a simple Wireshark capture would have cut to the heart of the issue.
I know there are some hard core, command line elitists out there that swear by tcpdump or tshark, but my response to them is a resounding, "PLEASE!" When I'm busting the proverbial brain cell over my connectivity issues, I just don't have the time to parse through the many intricacies of command line output.
Anyway, I open this up to the community. Your thoughts on Wireshark and the incalculable value it adds to a Local Area Network.